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.

src/server/api/routers/lineage/auth.test.ts

@@ -50,7 +50,10 @@ mock.module("~/env/server", () => ({
     TURSO_LINEAGE_TOKEN: "test-token",
     TURSO_DB_API_TOKEN: "test-token",
     NESSA_DB_URL: "libsql://nessa-test.turso.io",
-    NESSA_DB_TOKEN: "test-token"
+    NESSA_DB_TOKEN: "test-token",
+    // Clerk env vars (required after migration in task 02)
+    NESSA_CLERK_SECRET: "sk_test_test-secret",
+    NESSA_CLERK_JWT_ISSUER: "https://nessa-test.clerk.accounts.dev"
   },
   validateServerEnv: () => ({}),
   isMissingEnvVar: () => false,