feat: add synthetic gateway keys (#27170)

> Mux is working on behalf of Mike.

## Summary

Add a per-user synthetic API key for chatd AI Gateway attribution. Chatd
resolves the key from the chat owner, extends it before expiry, and
discards the generated bearer token so the key is never a usable
credential.

There is no mapping table. The key is resolved from `api_keys` by a
deterministic token name (`chatd_<owner_id>_session_token`), mirroring
the provisionerd session token model, with three deltas that chatd
needs:

- **Login type guard**: token names are unvalidated user input, so a
user can create a bearer token with the colliding name. The lookup
excludes `login_type = 'token'` rows, so chatd never picks up (or
extends) a real user token. Synthetic keys are minted with the owner's
login type, which is never `token`.
- **In-place expiry extension instead of delete-and-reinsert**: chat
generations have no stop boundary, and an in-flight generation may have
already delegated the current key ID to aibridged. Extending
`expires_at` keeps the key ID stable forever.
- **Advisory-lock mint**: the unique index on token names is partial
(`WHERE login_type = 'token'`), so nothing DB-enforces uniqueness for
synthetic keys. A per-user advisory lock serializes concurrent mints.

Keys carry a minimal scope (`api_key:read`) as defense in depth; the
delegated gateway path never evaluates scopes and the secret is
discarded at mint.

Migration 000544 removes the foreign keys from the legacy message and
queue `api_key_id` columns while chatd continues stamping them for
rolling compatibility. Stale IDs are tolerated because routing uses
`chats.owner_id`. Individual key deletion, delete-all, and password
reset remove the key without changing chat history or queue versions,
and the next lookup remints it. Suspension does not delete the key;
delegated gateway authorization rejects inactive users at request time.

This is the first PR in a three-PR rollout and must be fully deployed
before #27171.

Refs
https://linear.app/codercom/issue/CODAGT-561/maintain-synthetic-api-key-per-user-per-chat

coderd/database/queries.sql.go

@@ -3163,6 +3163,51 @@ func (q *sqlQuerier) GetAPIKeysLastUsedAfter(ctx context.Context, lastUsed time.
 	return items, nil
 }
 
+const getChatGatewayAPIKey = `-- name: GetChatGatewayAPIKey :one
+SELECT
+	id, hashed_secret, user_id, last_used, expires_at, created_at, updated_at, login_type, lifetime_seconds, ip_address, token_name, scopes, allow_list
+FROM
+	api_keys
+WHERE
+	user_id = $1 AND
+	token_name = $2 AND
+	-- Token names are unvalidated user input, so a user could create a token
+	-- with the chat gateway name. Excluding login_type 'token' ensures chatd
+	-- never picks up (and extends) a real bearer token. Synthetic gateway
+	-- keys are minted with the owner's login type, which is never 'token'.
+	login_type != 'token'
+ORDER BY
+	created_at ASC, id ASC
+LIMIT
+	1
+`
+
+type GetChatGatewayAPIKeyParams struct {
+	UserID    uuid.UUID `db:"user_id" json:"user_id"`
+	TokenName string    `db:"token_name" json:"token_name"`
+}
+
+func (q *sqlQuerier) GetChatGatewayAPIKey(ctx context.Context, arg GetChatGatewayAPIKeyParams) (APIKey, error) {
+	row := q.db.QueryRowContext(ctx, getChatGatewayAPIKey, arg.UserID, arg.TokenName)
+	var i APIKey
+	err := row.Scan(
+		&i.ID,
+		&i.HashedSecret,
+		&i.UserID,
+		&i.LastUsed,
+		&i.ExpiresAt,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+		&i.LoginType,
+		&i.LifetimeSeconds,
+		&i.IPAddress,
+		&i.TokenName,
+		&i.Scopes,
+		&i.AllowList,
+	)
+	return i, err
+}
+
 const insertAPIKey = `-- name: InsertAPIKey :one
 INSERT INTO
 	api_keys (
@@ -29857,6 +29902,40 @@ func (q *sqlQuerier) GetUserCount(ctx context.Context, includeSystem bool) (int6
 	return count, err
 }
 
+const getUserForChatSyntheticAPIKeyByID = `-- name: GetUserForChatSyntheticAPIKeyByID :one
+SELECT id, email, username, hashed_password, created_at, updated_at, status, rbac_roles, login_type, avatar_url, deleted, last_seen_at, quiet_hours_schedule, name, github_com_user_id, hashed_one_time_passcode, one_time_passcode_expires_at, is_system, is_service_account, chat_spend_limit_micros
+FROM users
+WHERE id = $1::uuid
+`
+
+func (q *sqlQuerier) GetUserForChatSyntheticAPIKeyByID(ctx context.Context, id uuid.UUID) (User, error) {
+	row := q.db.QueryRowContext(ctx, getUserForChatSyntheticAPIKeyByID, id)
+	var i User
+	err := row.Scan(
+		&i.ID,
+		&i.Email,
+		&i.Username,
+		&i.HashedPassword,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+		&i.Status,
+		&i.RBACRoles,
+		&i.LoginType,
+		&i.AvatarURL,
+		&i.Deleted,
+		&i.LastSeenAt,
+		&i.QuietHoursSchedule,
+		&i.Name,
+		&i.GithubComUserID,
+		&i.HashedOneTimePasscode,
+		&i.OneTimePasscodeExpiresAt,
+		&i.IsSystem,
+		&i.IsServiceAccount,
+		&i.ChatSpendLimitMicros,
+	)
+	return i, err
+}
+
 const getUserShellToolDisplayMode = `-- name: GetUserShellToolDisplayMode :one
 SELECT
 	value AS shell_tool_display_mode