feat(backend): Add database initialization and seed data scripts (Phase 3)

Implemented complete database initialization and seed data system for Docker development environment.

Changes:
- Enhanced init-db.sql with PostgreSQL extensions (uuid-ossp, pg_trgm, btree_gin)
- Created seed-data.sql with demo tenant, users, project, epics, stories, and tasks
- Updated docker-compose.yml to mount both initialization scripts
- Added DEMO-ACCOUNTS.md documentation with credentials and testing guide
- Added test-db-init.ps1 PowerShell script for testing initialization

Features:
- Automatic demo data creation on first startup
- 2 demo users (Owner and Developer with Demo@123456 password)
- 1 demo project with realistic Epic/Story/Task hierarchy
- Idempotent seed data (checks if data exists before inserting)
- Multi-tenant structure with proper TenantId isolation
- Detailed logging and error handling

Demo Accounts:
- owner@demo.com / Demo@123456 (Owner role)
- developer@demo.com / Demo@123456 (Member role)

Demo Project Data:
- Tenant: Demo Company
- Project: DEMO - Demo Project
- Epic: User Authentication System
- 2 Stories (Login Page, Registration Feature)
- 7 Tasks (various statuses: Done, InProgress, Todo)

Testing:
- Run: .\scripts\test-db-init.ps1
- Or: docker-compose down -v && docker-compose up -d

Documentation: See scripts/DEMO-ACCOUNTS.md for full details

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

scripts/init-db.sql

@@ -1,17 +1,29 @@
+-- ============================================
 -- ColaFlow Database Initialization Script
--- This script runs automatically when PostgreSQL container starts
+-- This script runs automatically when PostgreSQL container starts for the first time
+-- File: /docker-entrypoint-initdb.d/01-init-db.sql
+-- ============================================
 
 -- Enable required extensions
-CREATE EXTENSION IF NOT EXISTS "uuid-ossp";
-CREATE EXTENSION IF NOT EXISTS "pg_trgm";  -- For full-text search
+CREATE EXTENSION IF NOT EXISTS "uuid-ossp";       -- UUID generation functions
+CREATE EXTENSION IF NOT EXISTS "pg_trgm";         -- Full-text search support (trigram matching)
+CREATE EXTENSION IF NOT EXISTS "btree_gin";       -- GIN index optimization for multi-column queries
 
--- Create initial database schema
--- Note: Actual schema will be created by EF Core migrations
+-- Grant all privileges on database
+GRANT ALL PRIVILEGES ON DATABASE colaflow TO colaflow;
 
--- Create test user for development
--- Password: Test123! (BCrypt hashed)
+-- Output confirmation
 DO $$
 BEGIN
-    -- Add any initial seed data here if needed
-    RAISE NOTICE 'Database initialized successfully';
+    RAISE NOTICE '========================================';
+    RAISE NOTICE 'ColaFlow Database Initialized Successfully!';
+    RAISE NOTICE '========================================';
+    RAISE NOTICE 'Extensions installed:';
+    RAISE NOTICE '  - uuid-ossp (UUID generation)';
+    RAISE NOTICE '  - pg_trgm (Full-text search)';
+    RAISE NOTICE '  - btree_gin (Index optimization)';
+    RAISE NOTICE '';
+    RAISE NOTICE 'Next: EF Core migrations will create schema';
+    RAISE NOTICE 'Next: Seed data will populate demo accounts';
+    RAISE NOTICE '========================================';
 END $$;