Add configurable database path via environment variables

- Add DATABASE_PATH environment variable support in app.py and aerich_config.py
- DATABASE_PATH: For simple relative/absolute paths (default: database/raggr.db)
- DATABASE_URL: For full connection strings (overrides DATABASE_PATH if set)
- Create .env.example with all configuration options documented
- Maintains backward compatibility with default database location

Usage:
  # Use default path
  python app.py

  # Use custom path for development
  DATABASE_PATH=dev.db python app.py

  # Use full connection string
  DATABASE_URL=sqlite://custom/path.db python app.py

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

aerich_config.py

@@ -1,7 +1,12 @@
 import os
 
+# Database configuration with environment variable support
+# Use DATABASE_PATH for relative paths or DATABASE_URL for full connection strings
+DATABASE_PATH = os.getenv("DATABASE_PATH", "database/raggr.db")
+DATABASE_URL = os.getenv("DATABASE_URL", f"sqlite://{DATABASE_PATH}")
+
 TORTOISE_ORM = {
-    "connections": {"default": os.getenv("DATABASE_URL", "sqlite:///app/database/raggr.db")},
+    "connections": {"default": DATABASE_URL},
     "apps": {
         "models": {
             "models": [