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.

package.json

@@ -16,6 +16,7 @@
   "dependencies": {
     "@aws-sdk/client-s3": "^3.953.0",
     "@aws-sdk/s3-request-presigner": "^3.953.0",
+    "@clerk/backend": "^3.12.0",
     "@libsql/client": "^0.15.15",
     "@motionone/solid": "^10.16.4",
     "@solidjs/meta": "^0.29.4",
@@ -57,6 +58,7 @@
     "motion": "^12.23.26",
     "solid-js": "^1.9.5",
     "solid-tiptap": "^0.8.0",
+    "svix": "^1.98.0",
     "ua-parser-js": "^2.0.7",
     "uuid": "^13.0.0",
     "vinxi": "^0.5.7",