fix(db): use gen_random_uuid for PostgreSQL compatibility

[koistya]

Follow-up to #2131 that resolves migration failures for users without the pg_uuidv7 extension.

Database

Switches from uuid_generate_v7() to gen_random_uuid() for universal PostgreSQL support. The pg_uuidv7 extension isn't always available (especially in local dev or hosted environments outside Neon / Cloud SQL).

For projects that need time-ordered UUIDs (better index locality), the README documents alternatives:

// PostgreSQL 18+ (native)
id: text().primaryKey().default(sql`uuidv7()`);

// Earlier versions (requires pg_uuidv7 extension)
id: text().primaryKey().default(sql`uuid_generate_v7()`);

Also adds casing: "snake_case" to Drizzle config to ensure camelCase TypeScript fields map correctly to snake_case database columns.

OAuth Login Flow

Simplifies the post-OAuth redirect handling:

• Before: Complex state machine with returnUrl, redirect, loading spinners, and manual useEffect polling
• After: Single returnTo param validated via Zod schema, router.invalidate() handles session refresh

// Search params now sanitized at parse time
const searchSchema = z.object({
  returnTo: z.string().optional()
    .transform((val) => getSafeRedirectUrl(val))
    .catch(undefined),
});

// Post-login navigation simplified
async function handleSuccess() {
  await invalidateSession(queryClient);
  await router.invalidate();
  await router.navigate({ to: search.returnTo ?? "/" });
}

Migration

For existing deployments using uuid_generate_v7():

1. Option A: Keep existing data as-is—UUIDs are just strings, no migration needed
2. Option B: Run bun --filter @repo/db generate to create a fresh migration reflecting the new defaults

New installations will use gen_random_uuid() automatically.

Ref #2108 #2114