Add database-backed configuration (AppSetting model, ConfigService, settings API)

- Add AppSetting model for key-value settings storage in PostgreSQL
- Create ConfigService with DB-first, env-var-fallback resolution
- Add admin-only /api/v1/settings CRUD endpoints
- Update tasks.py to use ConfigService for SMTP config
- Seed default settings on application startup
- Add 24 unit tests for ConfigService (all passing)

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/ce88d4a8-d8c2-49b3-95a1-30592105a769

backend/app/models/database_models.py

@@ -430,3 +430,30 @@ class GmailCredential(Base):
 
     # Relationships
     user = relationship("User", backref="gmail_credential")
+
+
+class AppSetting(Base):
+    """
+    Application settings stored in the database.
+
+    Provides database-backed configuration that supplements or overrides
+    environment variable settings. Bootstrap settings (DATABASE_URL,
+    SECRET_KEY, ENCRYPTION_KEY) must still come from environment variables,
+    but all other settings can be managed via the database.
+    """
+
+    __tablename__ = "app_settings"
+
+    id = Column(Integer, primary_key=True, index=True)
+    key = Column(String(255), unique=True, nullable=False, index=True)
+    value = Column(Text, nullable=True)
+    value_type = Column(String(50), default="string")  # string, int, float, bool, json
+    description = Column(Text, nullable=True)
+    is_secret = Column(Boolean, default=False)
+    category = Column(String(100), nullable=True, index=True)
+
+    # Timestamps
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(
+        DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False
+    )