feat(backend): off-chain PostgreSQL schema, migrations, and DB module

[priscaenoch]

closes #75

this PR implemented the following changes:

• Add pg@^8.11.3 dependency and @types/pg@^8.11.6 dev dependency
• backend/src/db/schema.sql: canonical reference schema for all four tables
• backend/src/db/migrations/001_create_tables_up.sql: creates users, groups, members, transactions with UUIDs, FK ON DELETE CASCADE, UNIQUE constraints, CHECK (percentage > 0 AND percentage <= 100), and all documented indexes
• backend/src/db/migrations/001_create_tables_down.sql: drops tables in reverse dependency order
• backend/src/db/migrate.ts: CLI runner (node dist/db/migrate.js [down]); reads SQL files relative to the compiled dist directory, exposed via pnpm db:migrate / pnpm db:migrate:down scripts
• backend/src/db/index.ts: exports a shared pg.Pool and a generic query() helper for use by future service modules
• backend/src/db/tests/migrations.test.ts: migration smoke tests (up then assert tables/indexes then down then assert clean) plus constraint tests (duplicate wallet_address, duplicate tx_hash, out-of-range percentage, FK violation); all tests skip gracefully when DATABASE_URL is not set
• backend/.env.example: remove duplicate HORIZON_URL entry, add Docker quick-start comment for DATABASE_URL