feat: add addAccountByToken tRPC mutation with upsert logic and tests

Adds a new mutation that accepts an email + raw OAuth token and upserts
the account — creating it if it doesn't exist, updating credentials if
it does. Covers all four scenarios with unit tests (new, existing,
empty-email, empty-token validation).

docs/server-api.md

@@ -190,6 +190,7 @@ Each procedure uses `require*Repository(ctx)` helpers that throw `TRPCError(INTE
 | updateAccountAuth | mutation | Update credentials |
 | markAccountExhausted | mutation | Set exhaustion timer |
 | listProviderNames | query | Available provider names |
+| addAccountByToken | mutation | Upsert account from OAuth token; returns `{ upserted, account }` |
 
 ### Proposals
 | Procedure | Type | Description |