panel-redesign: security review of 2.0 - sessions belong to a user by id

Stage 3, step 2. Reviewed by Fable (read-only), fixed by Sonnet with a
failing test before each fix, checked by Opus. No Critical or High
finding: route authorization, tenant isolation by id, the /server/
subtree guard, output encoding and the origin check were found sound.

Fixed
  - M1: a session was keyed by user name and had no link to the user. A
    deleted or renamed user's cookie stayed alive (renewed on every hit,
    before the user was looked up) and became a session of whoever was
    later created under that name, with that account's role. The
    baseline now has sessions.user_id REFERENCES users(id) ON DELETE
    CASCADE; the principal is resolved by id and a session is renewed
    only after its user was found. RenameSession is gone.
  - M2: any user's password change deleted every other user's sessions.
    It ends that user's other sessions only.
  - L1: a password set on the user form ends that user's sessions (an
    administrator editing their own account keeps the current one).
  - L2: store, saslpasswd2 and Postfix reload errors were shown verbatim
    on the application form, the inbound upstream / recipients forms and
    the limit recalculation - pages a domain administrator reaches.
    Validation errors (app.ValidationError, inbound.ValidationError,
    store.ErrNotAutoRateLimit) keep their text; anything else is logged
    and shown as "Could not save ... Check the logs and try again."
  - L3: store.ListDMARCReports treated an empty domain list as "all". It
    takes a scope now (AllDomains or a list; an empty list is no rows),
    and the DMARC hub is open to the domain role as the plan says: the
    reports of the assigned domains, without the instance-wide ingest
    box. The menu entry was already shown to them and led to a 404.
  - I1: /static/ answered with a listing of the embedded assets. A path
    that is not a known asset is a 404.

Consequences
  - The global DMARC hub lists reports of domains that were deleted
    since, too.
  - dev/migrate1x (not in the tree) no longer carries sessions over:
    everyone signs in again after the transfer.
  - docs/schema-migrations.md: the sessions row.

Recorded, not changed - for the owner:
  - L4: the report-address select names the followed user and their
    address to every administrator of the domain, by default the global
    administrator's login name. The plan asks for it.
  - L5: a delegated inbound upstream may be any host and port, loopback
    and private ranges included - within the trusted-operator model; to
    be listed among the accepted risks in security.md.
  - L6: Account's profile is saved without the password; becomes a
    requirement once password-reset mails to users.email.
  - Still echoing an error's text: backup and domain import (global
    only), the domain form's validation messages.

Not checked by the review: anything needing the container (e2e, real
Postfix / DNS), CSP behaviour in a live browser.

  go test -count=1 ./internal/web/... ./internal/store/... ./internal/app/...
    ./internal/inbound/... ./internal/dmarc/... ./cmd/panel
      ok  (all eleven packages; dev/migrate1x also ok)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/app/validate.go

@@ -12,6 +12,18 @@ const (
 	maxLoginLen = 64
 )
 
+// ValidationError is a refusal of what was submitted: its text says what the
+// administrator must fix and is written to be shown on the page. Any other
+// error from this package (a failed write, a failed reload) is not, and the
+// caller must not put its text in front of a user.
+type ValidationError struct{ msg string }
+
+func (e *ValidationError) Error() string { return e.msg }
+
+func invalid(format string, args ...any) error {
+	return &ValidationError{msg: fmt.Sprintf(format, args...)}
+}
+
 // validateLogin enforces a strict server-side whitelist for the SASL login
 // (security.md). It intentionally excludes '@': the login is stored in sasldb2,
 // where '@' separates the user from the realm, so allowing it would change the
@@ -22,14 +34,14 @@ const (
 // makes that safe.
 func validateLogin(login string) error {
 	if len(login) < minLoginLen || len(login) > maxLoginLen {
-		return fmt.Errorf("login must be %d-%d characters", minLoginLen, maxLoginLen)
+		return invalid("login must be %d-%d characters", minLoginLen, maxLoginLen)
 	}
 	for _, r := range login {
 		lower := r >= 'a' && r <= 'z'
 		upper := r >= 'A' && r <= 'Z'
 		digit := r >= '0' && r <= '9'
 		if !lower && !upper && !digit && r != '.' && r != '-' && r != '_' {
-			return fmt.Errorf("login may contain only letters, digits, '.', '-' and '_'")
+			return invalid("login may contain only letters, digits, '.', '-' and '_'")
 		}
 	}
 	return nil
@@ -43,14 +55,14 @@ func validateLogin(login string) error {
 // (security.md).
 func validateImportedPassword(password string) error {
 	if password == "" {
-		return fmt.Errorf("imported application password is empty")
+		return invalid("imported application password is empty")
 	}
 	if len(password) > 1024 {
-		return fmt.Errorf("imported application password is too long")
+		return invalid("imported application password is too long")
 	}
 	for _, r := range password {
 		if r < 0x20 || r == 0x7f {
-			return fmt.Errorf("imported application password contains control characters")
+			return invalid("imported application password contains control characters")
 		}
 	}
 	return nil
@@ -59,7 +71,7 @@ func validateImportedPassword(password string) error {
 // validateAddressMode checks the submitted mode is one of the two known values.
 func validateAddressMode(mode string) error {
 	if mode != store.AddressModeWildcard && mode != store.AddressModeList {
-		return fmt.Errorf("invalid address mode")
+		return invalid("invalid address mode")
 	}
 	return nil
 }
@@ -79,11 +91,11 @@ func normalizeAddress(addr string) string {
 func validateSenderAddress(addr, domain string) error {
 	at := strings.LastIndexByte(addr, '@')
 	if at < 0 {
-		return fmt.Errorf("%q is not a valid email address", addr)
+		return invalid("%q is not a valid email address", addr)
 	}
 	local, host := addr[:at], addr[at+1:]
 	if host != domain {
-		return fmt.Errorf("%q does not belong to domain %s", addr, domain)
+		return invalid("%q does not belong to domain %s", addr, domain)
 	}
 	if err := validateLocalPart(local); err != nil {
 		return fmt.Errorf("%q: %w", addr, err)
@@ -96,17 +108,17 @@ func validateSenderAddress(addr, domain string) error {
 // value is always safe to write verbatim into the Postfix map (security.md).
 func validateLocalPart(local string) error {
 	if local == "" {
-		return fmt.Errorf("missing the part before '@'")
+		return invalid("missing the part before '@'")
 	}
 	if local[0] == '.' || local[len(local)-1] == '.' {
-		return fmt.Errorf("local part must not start or end with '.'")
+		return invalid("local part must not start or end with '.'")
 	}
 	for i := 0; i < len(local); i++ {
 		c := local[i]
 		lower := c >= 'a' && c <= 'z'
 		digit := c >= '0' && c <= '9'
 		if !lower && !digit && c != '.' && c != '-' && c != '_' && c != '+' {
-			return fmt.Errorf("local part may contain only lower-case letters, digits, '.', '-', '_' and '+'")
+			return invalid("local part may contain only lower-case letters, digits, '.', '-', '_' and '+'")
 		}
 	}
 	return nil
@@ -134,7 +146,7 @@ func parseAddresses(raw []string, domain string) ([]string, error) {
 		out = append(out, addr)
 	}
 	if len(out) == 0 {
-		return nil, fmt.Errorf("list mode requires at least one address")
+		return nil, invalid("list mode requires at least one address")
 	}
 	return out, nil
 }