feat(db): migrate to PostgreSQL 16 standalone

- docker-compose.prod.yml: add postgres:16-alpine service with health check,
  dedicated prod_leopost_net, backup volume mount, connection pool
- requirements.txt: add psycopg2-binary==2.9.9
- database.py: remove SQLite-specific run_migrations(), add PG pool_size/
  max_overflow/pool_pre_ping, keep sqlite compat for dev
- main.py: remove run_migrations call, rely on create_all for PG
- scripts/migrate_sqlite_to_pg.py: one-shot data migration script

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/database.py

@@ -1,13 +1,21 @@
-from sqlalchemy import create_engine, text
+from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker, declarative_base
 
 from .config import settings
 
+# PostgreSQL: use connection pool; SQLite (dev/test): check_same_thread
 connect_args = {}
 if settings.database_url.startswith("sqlite"):
     connect_args["check_same_thread"] = False
+    engine = create_engine(settings.database_url, connect_args=connect_args)
+else:
+    engine = create_engine(
+        settings.database_url,
+        pool_size=10,
+        max_overflow=20,
+        pool_pre_ping=True,
+    )
 
-engine = create_engine(settings.database_url, connect_args=connect_args)
 SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
 Base = declarative_base()
 
@@ -18,69 +26,3 @@ def get_db():
         yield db
     finally:
         db.close()
-
-
-def run_migrations(engine):
-    """SQLite-safe migration: add new columns if they don't exist."""
-    migrations = {
-        "users": [
-            ("email", "VARCHAR"),
-            ("display_name", "VARCHAR"),
-            ("avatar_url", "VARCHAR"),
-            ("auth_provider", "VARCHAR DEFAULT 'local'"),
-            ("google_id", "VARCHAR"),
-            ("subscription_plan", "VARCHAR DEFAULT 'freemium'"),
-            ("subscription_expires_at", "DATETIME"),
-            ("is_admin", "BOOLEAN DEFAULT 0"),
-            ("posts_generated_this_month", "INTEGER DEFAULT 0"),
-            ("posts_reset_date", "DATE"),
-        ],
-        "characters": [("user_id", "INTEGER")],
-        "posts": [("user_id", "INTEGER")],
-        "affiliate_links": [("user_id", "INTEGER")],
-        "editorial_plans": [("user_id", "INTEGER")],
-        "social_accounts": [("user_id", "INTEGER")],
-        "system_settings": [("user_id", "INTEGER")],
-    }
-    with engine.connect() as conn:
-        for table, cols in migrations.items():
-            try:
-                existing = {row[1] for row in conn.execute(text(f"PRAGMA table_info({table})"))}
-                for col_name, col_def in cols:
-                    if col_name not in existing:
-                        conn.execute(text(f"ALTER TABLE {table} ADD COLUMN {col_name} {col_def}"))
-                conn.commit()
-            except Exception as e:
-                print(f"Migration warning for {table}: {e}")
-
-        # Fix system_settings: remove UNIQUE constraint on 'key' by recreating the table
-        # This allows per-user settings (same key, different user_id)
-        try:
-            indexes = list(conn.execute(text("PRAGMA index_list(system_settings)")))
-            has_unique_key = any(
-                row[1].lower().startswith("ix_") or "key" in row[1].lower()
-                for row in indexes
-                if row[2] == 1  # unique=1
-            )
-            # Check via table creation SQL
-            create_sql_row = conn.execute(text(
-                "SELECT sql FROM sqlite_master WHERE type='table' AND name='system_settings'"
-            )).fetchone()
-            if create_sql_row and "UNIQUE" in (create_sql_row[0] or "").upper():
-                # Recreate without UNIQUE on key
-                conn.execute(text("ALTER TABLE system_settings RENAME TO system_settings_old"))
-                conn.execute(text("""
-                    CREATE TABLE system_settings (
-                        id INTEGER PRIMARY KEY,
-                        key VARCHAR(100) NOT NULL,
-                        value JSON,
-                        updated_at DATETIME,
-                        user_id INTEGER REFERENCES users(id)
-                    )
-                """))
-                conn.execute(text("INSERT INTO system_settings SELECT id, key, value, updated_at, user_id FROM system_settings_old"))
-                conn.execute(text("DROP TABLE system_settings_old"))
-                conn.commit()
-                print("Migration: system_settings UNIQUE constraint on key removed.")
-        except Exception as e:
-            print(f"Migration warning for system_settings UNIQUE fix: {e}")