Add users table to Drizzle schema (task 2.1)

Adds the `users` table to `src/lib/db/schema.ts` with:
- UUID primary key (defaultRandom)
- email (unique, not null)
- password_hash (nullable, for OAuth-only users)
- name, image (nullable)
- provider (default 'credentials'), provider_account_id (nullable)
- email_verified (timestamp, nullable)
- created_at, updated_at

Marks task 2.1 as complete in tasks.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openspec/changes/user-accounts/tasks.md

@@ -6,7 +6,7 @@
 
 ## 2. Database Schema & Migration
 
-- [ ] 2.1 `[sonnet]` Add `users` table to Drizzle schema (`src/lib/db/schema.ts`) with UUID PK, email, password_hash, name, image, provider, provider_account_id, email_verified, created_at, updated_at
+- [x] 2.1 `[sonnet]` Add `users` table to Drizzle schema (`src/lib/db/schema.ts`) with UUID PK, email, password_hash, name, image, provider, provider_account_id, email_verified, created_at, updated_at
 - [ ] 2.2 `[sonnet]` Add `user_id` (uuid, FK to users.id) column to `charts`, `annotations`, `annotation_types`, `span_annotations`, `span_label_types` in schema
 - [ ] 2.3 `[sonnet]` Replace unique constraints: `charts.name` → `(user_id, name)`, `annotation_types.name` → `(user_id, name)`, `span_label_types.name` → `(user_id, name)`
 - [ ] 2.4 `[haiku]` Generate Drizzle migration with `drizzle-kit generate`