refactor: consolidate blockchain explorer into single app and update backup ignore patterns

- Remove standalone explorer-web app (README, HTML, package files)
- Add /web endpoint to blockchain-explorer for web interface access
- Update .gitignore to exclude application backup archives (*.tar.gz, *.zip)
- Add backup documentation files to .gitignore (BACKUP_INDEX.md, README.md)
- Consolidate explorer functionality into main blockchain-explorer application

apps/exchange/database.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+"""
+Database configuration for the AITBC Trade Exchange
+"""
+
+import os
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, Session
+from sqlalchemy.pool import StaticPool
+from models import Base
+
+# Database configuration
+DATABASE_URL = os.getenv("DATABASE_URL", "sqlite:///./exchange.db")
+
+# Create engine
+if DATABASE_URL.startswith("sqlite"):
+    engine = create_engine(
+        DATABASE_URL,
+        connect_args={"check_same_thread": False},
+        poolclass=StaticPool,
+        echo=False  # Set to True for SQL logging
+    )
+else:
+    engine = create_engine(DATABASE_URL, echo=False)
+
+# Create session factory
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+# Create tables
+def init_db():
+    """Initialize database tables"""
+    Base.metadata.create_all(bind=engine)
+
+def get_db() -> Session:
+    """Get database session"""
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
+
+# Dependency for FastAPI
+def get_db_session():
+    """Get database session for FastAPI dependency"""
+    db = SessionLocal()
+    try:
+        return db
+    finally:
+        pass  # Don't close here, let the caller handle it