fix: remove SQLite fallback code from db_engine.py (#70)

Removes DATABASE_TYPE conditional entirely. PostgreSQL is now the only
supported backend. Moves PooledPostgresqlDatabase import to top-level
and raises RuntimeError at startup if POSTGRES_PASSWORD is unset,
preventing silent misconnection with misleading errors.

Closes #70

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

app/db_engine.py

@@ -8,35 +8,25 @@ from typing import Literal, List, Optional
 from pandas import DataFrame
 from peewee import *
 from peewee import ModelSelect
+from playhouse.pool import PooledPostgresqlDatabase
 from playhouse.shortcuts import model_to_dict
 
-# Database configuration - supports both SQLite and PostgreSQL
-DATABASE_TYPE = os.environ.get('DATABASE_TYPE', 'sqlite')
+_postgres_password = os.environ.get('POSTGRES_PASSWORD')
+if _postgres_password is None:
+    raise RuntimeError('POSTGRES_PASSWORD environment variable is required')
 
-if DATABASE_TYPE.lower() == 'postgresql':
-    from playhouse.pool import PooledPostgresqlDatabase
-    db = PooledPostgresqlDatabase(
-        os.environ.get('POSTGRES_DB', 'sba_master'),
-        user=os.environ.get('POSTGRES_USER', 'sba_admin'),
-        password=os.environ.get('POSTGRES_PASSWORD', 'sba_dev_password_2024'),
-        host=os.environ.get('POSTGRES_HOST', 'sba_postgres'),
-        port=int(os.environ.get('POSTGRES_PORT', '5432')),
-        max_connections=20,
-        stale_timeout=300,  # 5 minutes
-        timeout=0,
-        autoconnect=True,
-        autorollback=True  # Automatically rollback failed transactions
-    )
-else:
-    # Default SQLite configuration
-    db = SqliteDatabase(
-        'storage/sba_master.db',
-        pragmas={
-            'journal_mode': 'wal',
-            'cache_size': -1 * 64000,
-            'synchronous': 0
-        }
-    )
+db = PooledPostgresqlDatabase(
+    os.environ.get('POSTGRES_DB', 'sba_master'),
+    user=os.environ.get('POSTGRES_USER', 'sba_admin'),
+    password=_postgres_password,
+    host=os.environ.get('POSTGRES_HOST', 'sba_postgres'),
+    port=int(os.environ.get('POSTGRES_PORT', '5432')),
+    max_connections=20,
+    stale_timeout=300,  # 5 minutes
+    timeout=0,
+    autoconnect=True,
+    autorollback=True  # Automatically rollback failed transactions
+)
 
 date = f'{datetime.datetime.now().year}-{datetime.datetime.now().month}-{datetime.datetime.now().day}'
 logger = logging.getLogger('discord_app')