analytics: fix the two FRE-369 findings in the funnel instrumentation

The security review of 39d874a (FRE-369) found the address fix recorded
the wrong thing twice. Both are fixed here; the branch is the same one
under review, this commit does not rewrite it.

Q1 — the first x-forwarded-for hop is attacker-chosen text. In
client → Cloudflare → same-host proxy → app, both proxies *append*, so
the header arrives as `<client spoof…>, <true client>, <CF edge>` and
`split(",")[0]` took the sender's own invention — the loopback-peer gate
was sound, the hop selection defeated it. Preference is now
`cf-connecting-ip` (Cloudflare overwrites it, so a client value never
survives the edge), then x-forwarded-for counted from the right with one
trusted hop discounted, then the direct peer. Every candidate must parse
as an address. h3's `getRequestIP(…, { xForwardedFor: true })` has the
same leftmost flaw and is documented as not a fix.

Q4 — the table held full client addresses indefinitely. Every analytics
entry now passes through `truncateIpAddress` in `enrichAnalyticsEntry`
(IPv4 /24, IPv6 /56, unparseable dropped), so no writer can persist a
full address by accident. Retention got the schedule it never had:
`/api/analytics/retention` on a daily Vercel cron, Bearer-authenticated
and fail-closed, with the window parameterised (the old
`cleanupOldAnalytics` spliced the number into the SQL). Default 365 days
rather than the review's 90 so enabling the cron is not itself a
destructive event — the tighter window is an operational decision, named
on the task.

Also from the review: the module doc now forbids feeding this helper into
any security decision; `isLoopbackAddress` parses instead of
string-matching (`::ffff:127.9.9.9`, `0:0:0:0:0:0:0:1`); and `host` is
normalised (lowercased, port stripped) so `nook.freno.me:443` cannot
silently miss the `host = ?` scope. The weekly loopback caveat matches
the truncated shapes writers produce now, and says plainly that "unique
visitors" counts prefixes.

src/env/server.ts

@@ -69,6 +69,12 @@ const serverEnvSchema = z.object({
   // Bearer secret for the daily lifecycle-email cron. Falls back to
   // Vercel's CRON_SECRET; unset + no CRON_SECRET = the endpoint denies all.
   NOOK_LIFECYCLE_SECRET: z.string().min(1).optional(),
+  // Daily VisitorAnalytics retention cron (/api/analytics-retention). Same
+  // shape: falls back to CRON_SECRET, and unset denies every request.
+  ANALYTICS_RETENTION_SECRET: z.string().min(1).optional(),
+  // How long a VisitorAnalytics row lives. Unset (or unusable) = 365 days;
+  // see src/server/analytics-retention.ts before shortening it.
+  ANALYTICS_RETENTION_DAYS: z.string().optional(),
   CRON_SECRET: z.string().min(1).optional()
 });