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/meta/_journal.json

@@ -5,8 +5,8 @@
     {
       "idx": 0,
       "version": "6",
-      "when": 1781696310538,
-      "tag": "0000_youthful_genesis",
+      "when": 1781697895207,
+      "tag": "0000_stiff_captain_britain",
       "breakpoints": true
     }
   ]