orpc-v2: migrate auth plugin to the new plugin API

[elliotBraem]

Ticket ID: B4b

Parent

#248 — orpc-v2: consolidate fork into everything-dev and migrate platform to oRPC v2 + Effect 4 (epic)

What to build

Stage B, step 4b: migrate the auth plugin (SIWN sign-in, ephemeral relayer, organizations, API keys, account management) to the new createPlugin API — Layer-based initialize for its db layer, migrations, and session stores; no context-bridge module. The relayer's ephemeral key recovery must survive the restructure (same keypair recovered across restarts).

Acceptance criteria

• Auth plugin test suite green on the new API
• Sign-in / sign-out / session / API-key regression flows green
• Relayer key recovery verified across restart; relay endpoints functional

Blocked by

• orpc-v2: createPlugin restructure (Layer-based initialize, errorStatusMap, scope lifecycle) #259 — createPlugin restructure
• orpc-v2: migrate all six contracts to the v2 route builder #260 — migrate all six contracts to the v2 route builder