fix(coderd)!: restrict OIDC email fallback to first-time account linking (#25712)

## Problem

`findLinkedUser` in `coderd/userauth.go` falls back to email-based user
lookup when no `linked_id` match is found. This fallback was used for
**all logins**, not just first-time linking. An attacker who registers
the victim's email at the IdP (with a different OIDC subject) bypasses
the `linked_id` check and gets matched to the victim's Coder account.

Combined with the `email_verified` type assertion bypass (PLAT-228),
this creates a chained account-takeover vector.

## Fix

Restrict the email fallback in `findLinkedUser` so that when a user
found by email already has a `user_link` with a non-empty `linked_id`
that **differs** from the current login's `linked_id`, the function
returns no user. This blocks account takeover while preserving:

- **First-time linking**: No existing `user_link` exists, email fallback
works as before.
- **Legacy links**: Empty `linked_id` (pre-migration), email fallback
still works.
- **Normal logins**: Matching `linked_id` resolves via the primary path,
no fallback needed.

Also adds a `UpdateUserLinkedID` query to backfill `linked_id` on legacy
links (only when currently empty) during login, gradually migrating them
to the secure path.

The `findLinkedUser` signature now accepts `loginType` explicitly
instead of relying on `user.LoginType`, ensuring the correct link is
checked in the legacy lookup.

## Breaking change

Marked `release/breaking`. An account whose `user_link` already has a
populated `linked_id` that does not match the subject the IdP presents
will now be denied login (403) instead of silently resolving via the
email fallback. The most likely trigger is changing
`CODER_OIDC_ISSUER_URL` (the `linked_id` is `issuer||subject`), or two
identities sharing one email. Accounts with an empty (legacy)
`linked_id` are unaffected and are backfilled on their next login.

Fixes: https://linear.app/codercom/issue/PLAT-229

<details><summary>Implementation details</summary>

### Files changed

- `coderd/userauth.go`: Core fix in `findLinkedUser` + backfill logic in
`oauthLogin`
- `coderd/database/queries/user_links.sql`: New `UpdateUserLinkedID`
query
- `coderd/database/dbauthz/dbauthz.go`: Authorization for new query
(`ActionUpdate` on the user object, matching `InsertUserLink`)
- `coderd/userauth_test.go`: New OIDC and GitHub tests
- Generated files: `queries.sql.go`, `querier.go`, `dbmock.go`,
`querymetrics.go`

### New tests

- `TestUserOIDC/OIDCEmailFallbackBlockedByExistingLink`: Attacker with a
different `sub` but the same email is rejected (403) when the victim has
an existing link (covers signups enabled and disabled).
- `TestUserOIDC/OIDCFirstTimeLinkByEmailAllowed`: User created via
SCIM/API (no `user_link`) can still link via email on first OIDC login,
and the `linked_id` is populated.
- `TestUserOIDC/OIDCLegacyLinkBackfill`: User with empty `linked_id` can
login and their `linked_id` is backfilled with the correct value.
- `TestUserOIDC/OIDCEmailFallbackBlockedByIssuerChange`: Existing link
recorded under a previous issuer is rejected (403) after the issuer
changes (documents the breaking behavior).
- `TestUserOAuth2Github/EmailFallbackBlockedByExistingLink`: GitHub
attacker with a different user ID but the victim's email is rejected
(403).

</details>

> [!NOTE]
> This PR was authored by Coder Agents on behalf of @f0ssel.

---------

Co-authored-by: Coder Agents <agents@coder.com>

coderd/database/queries.sql.go

@@ -27224,6 +27224,41 @@ func (q *sqlQuerier) UpdateUserLink(ctx context.Context, arg UpdateUserLinkParam
 	return i, err
 }
 
+const updateUserLinkedID = `-- name: UpdateUserLinkedID :one
+UPDATE
+	user_links
+SET
+	linked_id = $1
+WHERE
+	user_id = $2 AND login_type = $3 AND linked_id = '' RETURNING user_id, login_type, linked_id, oauth_access_token, oauth_refresh_token, oauth_expiry, oauth_access_token_key_id, oauth_refresh_token_key_id, claims
+`
+
+type UpdateUserLinkedIDParams struct {
+	LinkedID  string    `db:"linked_id" json:"linked_id"`
+	UserID    uuid.UUID `db:"user_id" json:"user_id"`
+	LoginType LoginType `db:"login_type" json:"login_type"`
+}
+
+// Backfills linked_id for legacy user_links that were created before
+// linked_id tracking was added. Only updates when linked_id is empty
+// to avoid overwriting a valid binding.
+func (q *sqlQuerier) UpdateUserLinkedID(ctx context.Context, arg UpdateUserLinkedIDParams) (UserLink, error) {
+	row := q.db.QueryRowContext(ctx, updateUserLinkedID, arg.LinkedID, arg.UserID, arg.LoginType)
+	var i UserLink
+	err := row.Scan(
+		&i.UserID,
+		&i.LoginType,
+		&i.LinkedID,
+		&i.OAuthAccessToken,
+		&i.OAuthRefreshToken,
+		&i.OAuthExpiry,
+		&i.OAuthAccessTokenKeyID,
+		&i.OAuthRefreshTokenKeyID,
+		&i.Claims,
+	)
+	return i, err
+}
+
 const createUserSecret = `-- name: CreateUserSecret :one
 INSERT INTO user_secrets (
     id,