Phase 4: applications + SASL (sasldb2) + sender_login_maps

Adds application accounts bound to domains: a SASL login/password in
sasldb2, a per-application address mode (wildcard @domain or an explicit
list), and matching smtpd_sender_login_maps bindings — with create,
list, edit-mode, delete and password regeneration (spec 4.1, 5.1,
7.2.5-9). Generated passwords are shown exactly once and never stored in
plaintext (7.6.1).

- internal/store/applications.go: transactional CRUD; globally unique
  login; ListBindings (address->login) as the map source; logins-by-
  domain for pre-cascade SASL cleanup.
- internal/app: saslpasswd2 wrapper (password via stdin, login as a
  whitelisted argv element, no shell — 7.6.3); strong base64url password;
  address validation that enforces domain ownership before any config
  write (7.6.2); service orchestrating store + sasldb2 + map with full
  rollback on partial failure.
- internal/postfix: sender_login_maps regenerated as a pure function of
  the registry (many-to-one logins merged per address), atomic write,
  injection backstop (7.6.4).
- Postfix reload, corrected: `postfix start-fg` forks a separate master,
  so signalling the supervised process never reaches it. Reload now runs
  the canonical `postfix reload` via a one-shot supervisord program the
  unprivileged panel triggers over the group control socket. Verified in
  mail.log.
- domain.Service.Delete purges the domain's SASL accounts, then cascades,
  then rebuilds the sender map and reloads; manual reload now covers both
  OpenDKIM and Postfix.
- web: application management in the domain page, one-time credential
  shown inline; postfix joins the selfpost group and entrypoint normalises
  /data/sasl and /data/postfix (setgid, group-readable) with self-heal.

Verified on the dev server: gofmt/vet/test green, image builds, and a
container e2e covers the full application lifecycle, domain-delete
cascade, restart persistence, and a real postfix reload.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/app/validate.go

@@ -0,0 +1,119 @@
+package app
+
+import (
+	"fmt"
+	"strings"
+
+	"codeberg.org/mix/selfpost/internal/store"
+)
+
+const (
+	minLoginLen = 3
+	maxLoginLen = 64
+)
+
+// validateLogin enforces a strict server-side whitelist for the SASL login
+// (spec 7.6.2). It intentionally excludes '@': the login is stored in sasldb2,
+// where '@' separates the user from the realm, so allowing it would change the
+// account's identity. Client validation is never trusted.
+//
+// The login is the one piece of user input that is passed to saslpasswd2 as a
+// command argument (never through a shell, spec 7.6.3); this whitelist is what
+// 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)
+	}
+	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 nil
+}
+
+// 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 nil
+}
+
+// normalizeAddress lower-cases and trims a sender address. Both the local part
+// and domain are treated case-insensitively for the ownership check and for the
+// generated map, matching how addresses are compared elsewhere.
+func normalizeAddress(addr string) string {
+	return strings.ToLower(strings.TrimSpace(addr))
+}
+
+// validateSenderAddress enforces that a list-mode address is well-formed and,
+// critically, belongs to the application's own domain (spec 7.6.2). The domain
+// check is done here, before anything is written to a config file — not left to
+// smtpd_sender_login_maps to catch at delivery time. domain must already be a
+// validated, normalised domain name.
+func validateSenderAddress(addr, domain string) error {
+	at := strings.LastIndexByte(addr, '@')
+	if at < 0 {
+		return fmt.Errorf("%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)
+	}
+	if err := validateLocalPart(local); err != nil {
+		return fmt.Errorf("%q: %w", addr, err)
+	}
+	return nil
+}
+
+// validateLocalPart applies a conservative whitelist to the part before '@'.
+// This is deliberately stricter than RFC 5321 (no quoted local parts) so the
+// value is always safe to write verbatim into the Postfix map (spec 7.6.4).
+func validateLocalPart(local string) error {
+	if local == "" {
+		return fmt.Errorf("missing the part before '@'")
+	}
+	if local[0] == '.' || local[len(local)-1] == '.' {
+		return fmt.Errorf("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 nil
+}
+
+// parseAddresses normalises, validates and de-duplicates a list of submitted
+// sender addresses for a list-mode application. It requires at least one address
+// and that each belongs to domain. The returned slice is de-duplicated but keeps
+// submission order stable for display; the store sorts on read.
+func parseAddresses(raw []string, domain string) ([]string, error) {
+	seen := make(map[string]bool)
+	out := make([]string, 0, len(raw))
+	for _, r := range raw {
+		addr := normalizeAddress(r)
+		if addr == "" {
+			continue
+		}
+		if err := validateSenderAddress(addr, domain); err != nil {
+			return nil, err
+		}
+		if seen[addr] {
+			continue
+		}
+		seen[addr] = true
+		out = append(out, addr)
+	}
+	if len(out) == 0 {
+		return nil, fmt.Errorf("list mode requires at least one address")
+	}
+	return out, nil
+}