feat: add versioned migrations and kysely query layer

backend/libs/database/src/index.ts

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+export * from './schema';
+export * from './kysely.module';

backend/libs/database/src/kysely.module.ts

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+import { Module } from '@nestjs/common';
+import { Kysely, PostgresDialect } from 'kysely';
+import type { Pool } from 'pg';
+import { PostgresModule } from '../../infrastructure/src';
+import { POSTGRES_POOL } from '../../infrastructure/src';
+import type { Database } from './schema';
+
+export const KYSELY_DB = Symbol('KYSELY_DB');
+
+export const kyselyDbProvider = {
+  provide: KYSELY_DB,
+  inject: [POSTGRES_POOL],
+  useFactory: (pool: Pool) =>
+    new Kysely<Database>({ dialect: new PostgresDialect({ pool }) }),
+};
+
+@Module({
+  imports: [PostgresModule],
+  providers: [kyselyDbProvider],
+  exports: [kyselyDbProvider],
+})
+export class DatabaseModule {}

backend/libs/database/src/schema.ts

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+import type { ColumnType, Generated } from 'kysely';
+
+export interface UsersTable {
+  id: Generated<string>;
+  external_subject_id: string;
+  display_name: string;
+  email: string;
+  created_at: ColumnType<Date, string | undefined, never>;
+  updated_at: ColumnType<Date, string | undefined, string>;
+}
+
+export interface UserPreferencesTable {
+  user_id: string;
+  preferred_pace: string | null;
+  preferred_budget_level: string | null;
+  max_walking_distance_km: number | null;
+  preferred_start_time: string | null;
+  child_friendly_preferred: boolean;
+  interests: string[];
+  notes: string | null;
+  created_at: ColumnType<Date, string | undefined, never>;
+  updated_at: ColumnType<Date, string | undefined, string>;
+}
+
+export interface TripsTable {
+  id: Generated<string>;
+  name: string;
+  description: string | null;
+  owner_id: string;
+  start_date: string | null;
+  end_date: string | null;
+  status: string;
+  planning_stage: string | null;
+  currency: string;
+  version: Generated<number>;
+  created_at: ColumnType<Date, string | undefined, never>;
+  updated_at: ColumnType<Date, string | undefined, string>;
+}
+
+export interface TripSettingsTable {
+  trip_id: string;
+  web_research_enabled: boolean;
+  periodic_agent_review_enabled: boolean;
+  notification_email_enabled: boolean;
+  notification_push_enabled: boolean;
+  default_research_depth: string | null;
+  default_planning_style: string | null;
+  created_at: ColumnType<Date, string | undefined, never>;
+  updated_at: ColumnType<Date, string | undefined, string>;
+}
+
+export interface TripMembersTable {
+  id: Generated<string>;
+  trip_id: string;
+  user_id: string;
+  role: string;
+  status: string;
+  joined_at: ColumnType<Date, string | undefined, string> | null;
+  created_at: ColumnType<Date, string | undefined, never>;
+  updated_at: ColumnType<Date, string | undefined, string>;
+}
+
+export interface TripInvitationsTable {
+  id: Generated<string>;
+  trip_id: string;
+  email: string;
+  invited_by_user_id: string;
+  token_hash: string;
+  expires_at: ColumnType<Date, string, string>;
+  accepted_at: ColumnType<Date, string | undefined, string> | null;
+  created_at: ColumnType<Date, string | undefined, never>;
+  updated_at: ColumnType<Date, string | undefined, string>;
+}
+
+export interface TravelersTable {
+  id: Generated<string>;
+  trip_id: string;
+  linked_user_id: string | null;
+  display_name: string;
+  traveler_type: string;
+  created_by_user_id: string;
+  created_at: ColumnType<Date, string | undefined, never>;
+  updated_at: ColumnType<Date, string | undefined, string>;
+}
+
+export interface TripPreferenceOverridesTable {
+  id: Generated<string>;
+  trip_id: string;
+  user_id: string | null;
+  traveler_id: string | null;
+  overrides: unknown;
+  created_at: ColumnType<Date, string | undefined, never>;
+  updated_at: ColumnType<Date, string | undefined, string>;
+}
+
+export interface Database {
+  users: UsersTable;
+  user_preferences: UserPreferencesTable;
+  trips: TripsTable;
+  trip_settings: TripSettingsTable;
+  trip_members: TripMembersTable;
+  trip_invitations: TripInvitationsTable;
+  travelers: TravelersTable;
+  trip_preference_overrides: TripPreferenceOverridesTable;
+}

backend/libs/database/test/migration-runner.integration-spec.ts

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+import { Pool } from 'pg';
+import { runMigrations } from '../../../apps/api/src/migration';
+
+describe('runMigrations (integration)', () => {
+  it('applies all pending migrations idempotently against a real database', async () => {
+    await runMigrations();
+    await runMigrations(); // must be safe to run twice
+
+    const pool = new Pool({ connectionString: process.env.DATABASE_URL });
+    try {
+      const tables = await pool.query<{ table_name: string }>(
+        `select table_name from information_schema.tables where table_schema = 'public'`,
+      );
+      const names = tables.rows.map((r) => r.table_name);
+      for (const expected of [
+        'users',
+        'user_preferences',
+        'trips',
+        'trip_settings',
+        'trip_members',
+        'trip_invitations',
+        'travelers',
+        'trip_preference_overrides',
+      ]) {
+        expect(names).toContain(expected);
+      }
+    } finally {
+      await pool.end();
+    }
+  });
+});