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/web/handlers/handlers_settings.go

@@ -102,6 +102,8 @@ func (h *Handlers) submitServerSettings(w http.ResponseWriter, r *http.Request)
 		}
 	}
 	// An unticked checkbox is not sent at all: absent is off.
+	// HTML Living Standard, form-control-infrastructure.html#constructing-entry-list:
+	// a checkbox whose checkedness is false is skipped.
 	notifications := r.PostFormValue("notify") != ""
 	if err := h.store.SetSendLogRetentionDays(days); err != nil {
 		logf("panel: settings: set send-log retention failed: %v", err)