fix: resolve broken alembic migrations (#62) (#63)

- Replace shell variable interpolation in .env.example with literal
  values, since .env files don't expand shell variables
- Fix alembic/env.py to read DATABASE_URL instead of SQLALCHEMY_DATABASE_URL
  to match the app's environment variable convention

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -6,12 +6,12 @@ POSTGRES_USER=tracker
 POSTGRES_PASSWORD=dev_password
 
 # Constructed database URL (for application use)
-DATABASE_URL=postgresql://${POSTGRES_USER}:${POSTGRES_PASSWORD}@${POSTGRES_HOST}:${POSTGRES_PORT}/${POSTGRES_DB}
+DATABASE_URL=postgresql://tracker:dev_password@localhost:5432/polymarket_tracker
 
 # Redis Configuration
 REDIS_HOST=localhost
 REDIS_PORT=6379
-REDIS_URL=redis://${REDIS_HOST}:${REDIS_PORT}
+REDIS_URL=redis://localhost:6379
 
 # Optional: Development tool ports
 ADMINER_PORT=8080