store, web: cite the source of what they rely on (assumptions part D)

Comments only, no behaviour changed. modernc.org/sqlite v1.60.1 (the
_pragma list on every connection, busy_timeout first; extended result
codes; a deferred BEGIN; multi-statement Exec) and SQLite 3.53.4's
documentation (transactions, WAL, foreign keys, NOCASE, upsert, julianday,
partial indexes); htmx 2.0.4 src/htmx.js; the Fetch and HTML standards for
Sec-Fetch-Site, Origin, SameSite, __Host-, CSP and defer order; Go's
html/template. Comments the source contradicts are corrected - the driver
does not serialise writes (SetMaxOpenConns(1) does), htmx:load fires per
swapped element, no-store does not keep a page out of history.

Left for their own commits: D-S3 a migration two processes can both run,
D-S20 recipient case, D-B3 Origin: null under Referrer-Policy: no-referrer,
D-H5/H6/H7/H9 (audit M11, stage 4).

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

internal/store/sessions.go

@@ -30,7 +30,10 @@ func (s *Store) CreateSession(tokenHash string, userID int64, expiresAt time.Tim
 
 // LookupSession returns the session for tokenHash, if any. It does not check
 // expiry itself — callers compare ExpiresAt against time.Now() and call
-// DeleteSession on an expired row, keeping the read side lock-free.
+// DeleteSession on an expired row, keeping the read side lock-free: in WAL
+// mode a reader takes no write lock and is not blocked by a writer
+// (https://sqlite.org/wal.html § 2.2). Within this process it still waits for
+// the one pooled connection (Open).
 func (s *Store) LookupSession(tokenHash string) (SessionRow, bool, error) {
 	var (
 		row       SessionRow
@@ -99,6 +102,8 @@ func (s *Store) DeleteUserSessions(userID int64) error {
 // abandoned session (cookie never presented again) does not sit in the table
 // forever. It piggybacks on session creation rather than running as its own
 // background loop.
+// expires_at is RFC3339 UTC text, so < compares moments (see
+// ListQueuedOlderThan).
 func (s *Store) DeleteExpiredSessions(now time.Time) (int64, error) {
 	res, err := s.db.Exec(`DELETE FROM sessions WHERE expires_at < ?`, now.UTC().Format(time.RFC3339))
 	if err != nil {