feat: migrate Nessa auth to Clerk session tokens (task 03)

- src/server/nessa-auth.ts: replace jose HS256 sign/verify with Clerk
  session JWT verification via @clerk/backend verifyToken (RS256/JWKS).
  signNessaToken removed — frontend now supplies Clerk session tokens.
- src/server/api/utils.ts: createTRPCContext verifies Clerk JWT, resolves
  ctx.nessaUserId via SELECT id FROM users WHERE clerkUserId=? on the
  shared NessaConnectionFactory. Lookup miss throws typed UNAUTHORIZED
  (webhook has not run yet). Invalid/expired tokens are swallowed; the
  enforceNessaUser middleware rejects null nessaUserId.
- src/server/api/routers/nessa-community-authz.test.ts: add clerkUserId
  lookup tests (seeded match, missing row, mismatched id, local≠clerk).
- src/server/nessa-auth.test.ts: verifyNessaToken unit tests with mocked
  @clerk/backend (valid sub, missing sub, malformed/expired/wrong-signature
  rejection) plus static audit that signNessaToken is gone.
- src/server/clerk-user-webhook.ts + src/routes/api/clerk-webhook.ts:
  Clerk user.created/user.updated webhook handler (Svix signature
  verification, idempotent upsert by clerkUserId, lazy ALTER TABLE
  migration) with full test suite.
- src/server/api/routers/nessa.ts: remove legacy register/login/google/
  apple sign-in mutations (Clerk is now the sole identity provider).
- src/env/server.ts: add NESSA_CLERK_SECRET, NESSA_CLERK_JWT_ISSUER,
  NESSA_CLERK_WEBHOOK_SECRET; NESSA_JWT_SECRET moved to optional.
- package.json: add @clerk/backend, svix; lineage/auth.test.ts and
  nessa-ownership.test.ts: add Clerk env vars to env mocks.
- .env.example: document Clerk config vars and rotation.
- delete nessa-google-oauth.test.ts (Google auth removed).

ctx.nessaUserId remains the local users.id — router bodies are untouched.

.env.example

@@ -57,7 +57,10 @@ TURSO_LINEAGE_URL="libsql://<lineage-db>.turso.io"
 TURSO_LINEAGE_TOKEN="<rotate-in-turso-dashboard>"
 NESSA_DB_URL="libsql://<nessa-db>.turso.io"
 NESSA_DB_TOKEN="<rotate-in-turso-dashboard>"
-NESSA_CLERK_SECRET=<rotate-in-clerk-dashboard>
+# Clerk authentication — rotate via Clerk Dashboard or `clerk api .../rotate_secret_keys`
+NESSA_CLERK_SECRET="sk_test_<rotate-in-clerk-dashboard>"   # secret key (sk_test_... for dev, sk_live_... for prod)
+NESSA_CLERK_JWT_ISSUER="https://<your-app>.clerk.accounts.dev"  # JWT issuer from Clerk Dashboard
+NESSA_CLERK_WEBHOOK_SECRET="whsec_<rotate-in-clerk-dashboard>"  # Svix signing secret from Clerk Dashboard → Webhooks → Signing Secret
 
 APPLE_CLIENT_ID_LINEAGE=com...
 APPLE_CLIENT_ID_NESSA=com...