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/notify.go

@@ -137,6 +137,9 @@ func (s *Store) DeleteNotifyState(key string) error {
 
 // ListNotifyStateKeys returns the keys that begin with prefix, so that the
 // states of subjects that are gone - a deleted domain - can be found.
+// SQLite's substr counts characters of a TEXT value, len(prefix) its bytes
+// (https://sqlite.org/lang_corefunc.html#substr): the two agree for the ASCII
+// prefixes the callers pass ("dns:", "limit:").
 func (s *Store) ListNotifyStateKeys(prefix string) ([]string, error) {
 	rows, err := s.db.Query(
 		"SELECT key FROM notify_state WHERE substr(key, 1, ?) = ? ORDER BY key", len(prefix), prefix)
@@ -181,7 +184,9 @@ type LimitRefusal struct {
 // reason and is still judged the old way.
 func (s *Store) LimitRefusalsSince(since time.Time) ([]LimitRefusal, error) {
 	// The status is a literal so that the partial index of migration 0005
-	// answers the question.
+	// answers the question: the query's term matches the index's WHERE
+	// exactly (https://sqlite.org/partialindex.html § 3) when the statement is
+	// first prepared.
 	rows, err := s.db.Query(
 		`SELECT d.name, a.login, COUNT(*), MIN(l.created_at)
 		 FROM send_log l