fix(api): reconcile better-auth schema with installed better-auth CLI output

The Task 3 schema drifted from what better-auth@1.6.18's CLI generates: it used
mode:'timestamp' (epoch seconds) instead of 'timestamp_ms', left verification
timestamps nullable, and omitted the default expressions and helper indexes.

Regenerated src/db/schema.ts from `@better-auth/cli generate` (authoritative per
the plan's version-drift rule) and rebuilt migration 0000 from scratch (no data
exists yet). Converted the index callbacks from the CLI's array form to the
object form required by drizzle-orm@0.36.4's types. Adds session_userId_idx,
account_userId_idx, verification_identifier_idx and the unixepoch defaults.

Tests (health, db, auth sign-up/sign-in, /api/me round-trip) all pass; typecheck
clean; db:generate reports no pending changes.

apps/api/drizzle/0000_stiff_captain_britain.sql

@@ -0,0 +1,53 @@
+CREATE TABLE `account` (
+	`id` text PRIMARY KEY NOT NULL,
+	`account_id` text NOT NULL,
+	`provider_id` text NOT NULL,
+	`user_id` text NOT NULL,
+	`access_token` text,
+	`refresh_token` text,
+	`id_token` text,
+	`access_token_expires_at` integer,
+	`refresh_token_expires_at` integer,
+	`scope` text,
+	`password` text,
+	`created_at` integer DEFAULT (cast(unixepoch('subsecond') * 1000 as integer)) NOT NULL,
+	`updated_at` integer NOT NULL,
+	FOREIGN KEY (`user_id`) REFERENCES `user`(`id`) ON UPDATE no action ON DELETE cascade
+);
+--> statement-breakpoint
+CREATE INDEX `account_userId_idx` ON `account` (`user_id`);--> statement-breakpoint
+CREATE TABLE `session` (
+	`id` text PRIMARY KEY NOT NULL,
+	`expires_at` integer NOT NULL,
+	`token` text NOT NULL,
+	`created_at` integer DEFAULT (cast(unixepoch('subsecond') * 1000 as integer)) NOT NULL,
+	`updated_at` integer NOT NULL,
+	`ip_address` text,
+	`user_agent` text,
+	`user_id` text NOT NULL,
+	FOREIGN KEY (`user_id`) REFERENCES `user`(`id`) ON UPDATE no action ON DELETE cascade
+);
+--> statement-breakpoint
+CREATE UNIQUE INDEX `session_token_unique` ON `session` (`token`);--> statement-breakpoint
+CREATE INDEX `session_userId_idx` ON `session` (`user_id`);--> statement-breakpoint
+CREATE TABLE `user` (
+	`id` text PRIMARY KEY NOT NULL,
+	`name` text NOT NULL,
+	`email` text NOT NULL,
+	`email_verified` integer DEFAULT false NOT NULL,
+	`image` text,
+	`created_at` integer DEFAULT (cast(unixepoch('subsecond') * 1000 as integer)) NOT NULL,
+	`updated_at` integer DEFAULT (cast(unixepoch('subsecond') * 1000 as integer)) NOT NULL
+);
+--> statement-breakpoint
+CREATE UNIQUE INDEX `user_email_unique` ON `user` (`email`);--> statement-breakpoint
+CREATE TABLE `verification` (
+	`id` text PRIMARY KEY NOT NULL,
+	`identifier` text NOT NULL,
+	`value` text NOT NULL,
+	`expires_at` integer NOT NULL,
+	`created_at` integer DEFAULT (cast(unixepoch('subsecond') * 1000 as integer)) NOT NULL,
+	`updated_at` integer DEFAULT (cast(unixepoch('subsecond') * 1000 as integer)) NOT NULL
+);
+--> statement-breakpoint
+CREATE INDEX `verification_identifier_idx` ON `verification` (`identifier`);