fix: resolve database pathing issues in multi-environment setup

* Used absolute paths constructed from `app.root_path` (BASE_DIR)
* Modified `config.py` to ensure `sqlite:///{path}` works across Docker and Host Linux systems

config.py

@@ -38,6 +38,8 @@ import os
 from typing import Optional
 
 
+BASE_DIR = os.path.abspath(os.path.dirname(__file__))
+
 class Config:
     """
     Base configuration class with default values.
@@ -60,10 +62,11 @@ class Config:
     DEV_USER_EMAIL: str = os.environ.get('DEV_USER_EMAIL', 'dev@formeleon.local')
     
     # Database Configuration
-    SQLALCHEMY_DATABASE_URI: str = os.environ.get(
-        'SQLALCHEMY_DATABASE_URI', 
-        'sqlite:///formeleon.db'
-    )
+    if 'SQLALCHEMY_DATABASE_URI' in os.environ:
+        SQLALCHEMY_DATABASE_URI: str = os.environ['SQLALCHEMY_DATABASE_URI']
+    else:
+        SQLALCHEMY_DATABASE_URI: str = f'sqlite:///{os.path.join(BASE_DIR, "instance", "formeleon.db")}'
+
     SQLALCHEMY_TRACK_MODIFICATIONS: bool = False
     SQLALCHEMY_ECHO: bool = os.environ.get('SQLALCHEMY_ECHO', 'false').lower() == 'true'