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>

src/lib/db/schema.ts

@@ -1,4 +1,17 @@
-import { pgTable, serial, text, timestamp, doublePrecision, integer, boolean, jsonb, uniqueIndex } from 'drizzle-orm/pg-core';
+import { pgTable, serial, text, timestamp, doublePrecision, integer, boolean, jsonb, uniqueIndex, uuid } from 'drizzle-orm/pg-core';
+
+export const users = pgTable('users', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  email: text('email').notNull().unique(),
+  password_hash: text('password_hash'), // null for OAuth-only users
+  name: text('name'),
+  image: text('image'),
+  provider: text('provider').notNull().default('credentials'), // 'credentials' | 'google'
+  provider_account_id: text('provider_account_id'), // Google sub ID
+  email_verified: timestamp('email_verified'), // nullable — set when email is verified
+  created_at: timestamp('created_at').notNull().defaultNow(),
+  updated_at: timestamp('updated_at').notNull().defaultNow(),
+});
 
 export const charts = pgTable('charts', {
   id: serial('id').primaryKey(),