feat: Step 6 - Write-through cache API

Implement the core write-through cache pattern for node creation.
This is the architectural foundation of the application.

Changes:
- Add @google/generative-ai dependency for embeddings
- Create lib/db.ts: SurrealDB connection helper with JWT auth
- Create lib/ai.ts: AI embedding generation using text-embedding-004
- Create app/api/nodes/route.ts: POST endpoint implementing write-through cache

Write-through cache flow:
1. Authenticate user via SurrealDB JWT
2. Publish node to ATproto PDS (source of truth)
3. Generate 768-dimensional embedding via Google AI
4. Cache node + embedding + links in SurrealDB

Updated schema to use 768-dimensional embeddings (text-embedding-004)
instead of 1536 dimensions.

Security:
- Row-level permissions enforced via SurrealDB JWT
- All secrets server-side only
- ATproto OAuth tokens from secure cookies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/magnitude/03-auth.mag.ts

@@ -32,3 +32,23 @@ test('[Happy Path] User initiates OAuth flow', async (agent) => {
   // Note: Using http://localhost/ as client_id (per ATproto OAuth spec) allows local development.
   // See: https://atproto.com/specs/oauth#localhost-client-development
 });
+
+test('[Happy Path] User completes full login flow and sees their handle', async (agent) => {
+  await agent.act('Navigate to /login');
+  await agent.act(`Type "${TEST_HANDLE}" into the "Your Handle" input field`);
+  await agent.act('Click the "Log in with Bluesky" button');
+
+  // Wait to be redirected to Bluesky OAuth page
+  await agent.check('The page URL contains "bsky.social"');
+
+  // Log in to Bluesky (this will depend on the actual OAuth flow)
+  // The agent should handle the login form on Bluesky's side
+  await agent.act(`Type "${TEST_HANDLE}" into the username/identifier field`);
+  await agent.act(`Type "${TEST_PASSWORD}" into the password field`);
+  await agent.act('Click the submit/authorize button');
+
+  // After successful OAuth, we should be redirected back to /chat
+  // and see our Bluesky handle displayed on the page
+  await agent.check(`The text "${TEST_HANDLE}" is visible on the screen`);
+  await agent.check('The page URL contains "/chat"');
+});