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/sendercheck.go

@@ -100,6 +100,7 @@ func (s *Store) AddSenderCheckDeferred(at time.Time, n int64, place string, mess
 	if n <= 0 {
 		return nil
 	}
+	// The first statement is a write (see UpdateStatus).
 	tx, err := s.db.Begin()
 	if err != nil {
 		return fmt.Errorf("sender check deferred: %w", err)
@@ -118,7 +119,9 @@ func (s *Store) AddSenderCheckDeferred(at time.Time, n int64, place string, mess
 	}
 	// To the fraction of a second the log gives, and of a fixed width so that
 	// the strings compare as the times do; a re-read log must not move the
-	// mark back.
+	// mark back. SQLite 3.53.4, https://sqlite.org/lang_upsert.html § 2: a
+	// DO UPDATE whose WHERE is false is a no-op, not an error, and excluded.
+	// names the value the INSERT brought.
 	stamp := at.UTC().Format("2006-01-02T15:04:05.000000Z")
 	setting := func(key, value, cond string) error {
 		_, err := tx.Exec(`INSERT INTO settings (key, value) VALUES (?, ?)