…istic locking (#22904)

## Problem

When multiple concurrent callers (e.g., parallel workspace builds) read
the same single-use OAuth2 refresh token from the database and race to
exchange it with the provider, the first caller succeeds but subsequent
callers get `bad_refresh_token`. The losing caller then **clears the
valid new token** from the database, permanently breaking the auth link
until the user manually re-authenticates.

This is reliably reproducible when launching multiple workspaces
simultaneously with GitHub App external auth and user-to-server token
expiration enabled.

## Solution

Two layers of protection:

### 1. Singleflight deduplication (`Config.RefreshToken` +
`ObtainOIDCAccessToken`)

Concurrent callers for the same user/provider share a single refresh
call via `golang.org/x/sync/singleflight`, keyed by `userID`. The
singleflight callback re-reads the link from the database to pick up any
token already refreshed by a prior in-flight call, avoiding redundant
IDP round-trips entirely.

### 2. Optimistic locking on `UpdateExternalAuthLinkRefreshToken`

The SQL `WHERE` clause now includes `AND oauth_refresh_token =
@old_oauth_refresh_token`, so if two replicas (HA) race past
singleflight, the loser's destructive UPDATE is a harmless no-op rather
than overwriting the winner's valid token.

## Changes

| File | Change |
|------|--------|
| `coderd/externalauth/externalauth.go` | Added `singleflight.Group` to
`Config`; split `RefreshToken` into public wrapper +
`refreshTokenInner`; pass `OldOauthRefreshToken` to DB update |
| `coderd/provisionerdserver/provisionerdserver.go` | Wrapped OIDC
refresh in `ObtainOIDCAccessToken` with package-level singleflight |
| `coderd/database/queries/externalauth.sql` | Added optimistic lock
(`WHERE ... AND oauth_refresh_token = @old_oauth_refresh_token`) |
| `coderd/database/queries.sql.go` | Regenerated |
| `coderd/database/querier.go` | Regenerated |
| `coderd/database/dbauthz/dbauthz_test.go` | Updated test params for
new field |
| `coderd/externalauth/externalauth_test.go` | Added
`ConcurrentRefreshDedup` test; updated existing tests for singleflight
DB re-read |

## Testing

- **New test `ConcurrentRefreshDedup`**: 5 goroutines call
`RefreshToken` concurrently, asserts IDP refresh called exactly once,
all callers get same token.
- All existing `TestRefreshToken/*` subtests updated and passing.
- `TestObtainOIDCAccessToken` passing.
- `dbauthz` tests passing.

(cherry picked from commit 53e52ae)