fix(auth): carry a generated secret in the session cookie, not the row id

The cookie was the Session row's cuid. cuid is built for collision-
resistant identity across distributed writers, not for unguessability —
it encodes a millisecond timestamp and a process-local counter and never
touches a CSPRNG. A session cookie is a bearer credential and has to be
unguessable, so it now carries 32 random bytes and only the HMAC of that
value is stored, matching how api_tokens and refresh_tokens already hold
their secrets.

Compatibility is passive, and deliberately so. A backfill cannot help:
the cookie already sitting in a browser is the cuid, and nothing we
generate server-side can be pushed into it — backfilling would sign
everyone out at deploy. Upgrading a row in place on first use loses to
the parallel requests a single page load fires: each mints its own
secret, one write wins, and the losers cannot re-emit the winner's
secret because only a hash is stored, so the browser can end up holding
a value the row does not honour. Instead, rows minted before the column
keep a NULL token_hash and stay resolvable by id, and are denied the
sliding-expiry extension so they drain within the 30-day session
lifetime. Every login from this release forward is on a secret at once,
and an id stops authenticating the moment its row holds one.

Note for operators: API_TOKEN_HMAC_KEY now also keys session cookies, so
rotating it signs browser sessions out as well as invalidating tokens.
It is already Core-required, so no deployment gains a new prerequisite.

scripts/env-manifest.json

@@ -31,7 +31,7 @@
         },
         {
           "name": "API_TOKEN_HMAC_KEY",
-          "purpose": "HMAC-SHA256 key for hashing external Bearer tokens. 64 hex chars."
+          "purpose": "HMAC-SHA256 key for hashing external Bearer tokens AND web session cookies. 64 hex chars. Rotating it signs every browser session out on top of invalidating every issued token."
         }
       ]
     },