feat: pivot to island survival simulation with SQLite persistence

Phase 3 - "The Island" transformation:
- Add SQLAlchemy + SQLite for data persistence (database.py)
- Rewrite models.py with User, Agent, WorldState ORM models
- Refactor engine.py for survival mechanics (energy decay, starvation)
- Implement feed command (10 gold -> +20 energy)
- Auto-seed 3 NPCs on startup (Jack/Luna/Bob)
- Update frontend with agent card view and Chinese UI
- Remove old Boss/Player RPG mechanics
- Add .gitignore for database and cache files
- Fix SQLAlchemy session detachment issue

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/database.py

@@ -0,0 +1,61 @@
+"""
+Database configuration for The Island.
+SQLite + SQLAlchemy ORM setup.
+"""
+
+from pathlib import Path
+from contextlib import contextmanager
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, declarative_base
+
+# Database file path (in project root)
+DB_PATH = Path(__file__).parent.parent.parent / "island.db"
+DATABASE_URL = f"sqlite:///{DB_PATH}"
+
+# Create engine with SQLite optimizations
+engine = create_engine(
+    DATABASE_URL,
+    connect_args={"check_same_thread": False},  # Required for SQLite with FastAPI
+    echo=False  # Set True for SQL debugging
+)
+
+# Session factory
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+# Base class for ORM models
+Base = declarative_base()
+
+
+def init_db():
+    """Initialize database tables."""
+    Base.metadata.create_all(bind=engine)
+
+
+def get_db():
+    """
+    Dependency for getting database session.
+    Use with FastAPI's Depends() or as context manager.
+    """
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
+
+
+@contextmanager
+def get_db_session():
+    """
+    Context manager for database session.
+    Usage: with get_db_session() as db: ...
+    """
+    db = SessionLocal()
+    try:
+        yield db
+        db.commit()
+    except Exception:
+        db.rollback()
+        raise
+    finally:
+        db.close()