Add README and technical docs

- README with quick start, package overview, links to docs
- docs/setup.md — prerequisites, env vars, installation, running, testing
- docs/architecture.md — monorepo structure, backend/frontend design
- docs/api.md — full endpoint reference with permissions
- docs/database.md — schema overview, migrations, multi-tenancy
- docs/testing.md — test runner, suites, writing tests
- Updated .env.example with all supported variables

docs/database.md

@@ -0,0 +1,86 @@
+# Database
+
+## Setup
+
+PostgreSQL 16. Two databases:
+
+| Database | Port | Usage |
+|----------|------|-------|
+| `forte` | 5432 | Development |
+| `forte_api_test` | 5432 | API integration tests (auto-created by test runner) |
+
+## Migrations
+
+Migrations are managed by Drizzle Kit and live in `packages/backend/src/db/migrations/`.
+
+```bash
+cd packages/backend
+
+# Generate a migration from schema changes
+bunx drizzle-kit generate
+
+# Apply pending migrations
+bunx drizzle-kit migrate
+```
+
+Schema files: `packages/backend/src/db/schema/`
+
+## Multi-Tenancy
+
+All domain tables include `company_id` (uuid FK to `company`). Every query filters by the authenticated user's company. Location-scoped tables additionally include `location_id`.
+
+## Schema Overview
+
+### Core
+
+| Table | Description |
+|-------|-------------|
+| `company` | Tenant (music store business) |
+| `location` | Physical store location |
+| `user` | Staff/admin user account |
+
+### Accounts & Members
+
+| Table | Description |
+|-------|-------------|
+| `account` | Billing entity (family, individual, business) |
+| `member` | Individual person on an account |
+| `member_identifier` | ID documents (DL, passport, school ID) |
+| `payment_method` | Stored payment methods |
+| `processor_link` | Payment processor integrations |
+| `tax_exemption` | Tax exemption records |
+
+### Inventory
+
+| Table | Description |
+|-------|-------------|
+| `product` | Product catalog entry |
+| `inventory_unit` | Individual serialized/non-serialized unit |
+| `stock_receipt` | Incoming stock records |
+| `category` | Product categories |
+| `supplier` | Product suppliers |
+| `inventory_unit_status` | Lookup: unit statuses |
+| `item_condition` | Lookup: item conditions |
+
+### RBAC
+
+| Table | Description |
+|-------|-------------|
+| `permission` | System permissions (global, seeded) |
+| `role` | Company-scoped roles (system + custom) |
+| `role_permission` | Role-to-permission mapping |
+| `user_role_assignment` | User-to-role mapping |
+
+### Files
+
+| Table | Description |
+|-------|-------------|
+| `file` | File metadata (path, type, size, entity reference) |
+
+## Key Conventions
+
+- UUIDs for all primary keys (`defaultRandom()`)
+- `created_at` and `updated_at` timestamps with timezone on all tables
+- Soft deletes via `is_active` boolean where applicable
+- Auto-generated sequential numbers: `account_number` (6-digit), `member_number`
+- Lookup tables support both system (immutable) and custom (company-scoped) values