Add optional API key expiry and a missing way to list them

Part of the same security-hardening pass as the last five commits, and
the last item in its backlog. User API keys had no expiry at all --
unlike service-account keys, visibly distinct only by their "tdsa_"
prefix -- and, it turns out while implementing this, no way to list
them either: only create (returns the raw key once) and delete-by-id
existed, so a key's owner had no way to even discover what keys they
had short of remembering IDs from creation time.

handleCreateAPIKey takes an optional expires_in_days (0, the default,
keeps today's behavior: never expires, so no existing integration is
affected). apiKeyUser's lookup now carries `expires_at IS NULL OR
expires_at > now` as part of the query itself, the same way serveAs's
disabled_at check already works -- an expired key simply fails to
resolve, like a wrong one, rather than resolving and being caught
after the fact. New GET /api/users/{id}/api-keys (requireSelfOrAdmin,
same as create/delete) lists id/name/created_at/last_used_at/expires_at,
never the raw key.

Scoped down from the original plan on request: no web UI change, since
there turned out to be no existing API-keys UI at all to extend --
building one from scratch would have been a real feature addition, not
a hardening tweak.

Mirrored the additive expires_at field in terdut-tui's APIKey struct
(separate commit, separate repo) per this workspace's version-coupling
rule; the TUI does not create or list expiring keys itself yet.

New tests (api_keys_test.go): default never-expires, expires_in_days
sets expires_at, out-of-range values rejected, an expired key fails
auth after a fresh one worked, the listing never includes the raw key.
Also added the new GET route to authz_scope_test.go's self-or-admin
table from the previous commit.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/api/middleware.go

@@ -135,10 +135,16 @@ func requireSelfOrAdmin(w http.ResponseWriter, r *http.Request, targetID int64)
 }
 
 // apiKeyUser resolves an API key to its user and stamps its last use.
+// expires_at IS NULL OR > now is part of the lookup itself, the same way
+// serveAs's disabled_at check is: an expired key is one that cannot
+// authenticate, by construction, rather than one that happens to still
+// resolve and has to be caught afterwards.
 func apiKeyUser(ctx context.Context, db *sql.DB, token string) (int64, bool) {
 	var keyID, userID int64
 	err := db.QueryRowContext(ctx,
-		"SELECT id, user_id FROM api_keys WHERE key_hash = $1", hashToken(token),
+		`SELECT id, user_id FROM api_keys
+		  WHERE key_hash = $1 AND (expires_at IS NULL OR expires_at > $2)`,
+		hashToken(token), time.Now().Unix(),
 	).Scan(&keyID, &userID)
 	if err != nil {
 		return 0, false