docs: improve documentation across WebPushService class

Comprehensive documentation update focusing on clarity and completeness while
maintaining technical accuracy. Key improvements include:

- Add detailed module-level documentation with features and dependencies
- Enhance class-level documentation with responsibilities and endpoints
- Improve method documentation with:
  - Clear workflow descriptions
  - Technical implementation details
  - Security considerations
  - Database impacts
  - Error handling specifics
  - Usage examples
  - Return type clarification
  - Thread safety notes

Technical Changes:
- Replace print statements with structured logging using structlog
- Add specific error handling for SQLAlchemy and cryptography exceptions
- Add type hints and improve return type annotations

Security:
- Document authentication requirements
- Add security considerations sections
- Clarify VAPID key handling
- Document input validation

Dependencies:
- Add structlog>=24.1.0 to requirements.txt

This improves code maintainability and helps future developers understand
the system's security and operational characteristics.

SECURITY_CHECKLIST.md

@@ -0,0 +1,33 @@
+# Security Audit Checklist for Web Push Service
+
+## Authentication & Authorization
+- [x] Basic auth implemented for admin endpoints
+- [x] VAPID authentication for push notifications
+- [x] Environment variable for admin password
+- [ ] Consider rate limiting for subscription endpoints
+- [ ] Consider adding API key authentication for public endpoints
+
+## Data Validation
+- [x] Input validation for subscription data
+- [x] Message size limits (100 chars)
+- [x] Notification type validation
+- [ ] Consider adding input sanitization for messages
+
+## Database Security
+- [x] SQLite database with configurable path
+- [x] No raw SQL queries (uses SQLAlchemy ORM)
+- [ ] Consider adding database connection pooling
+- [ ] Consider encryption at rest for sensitive data
+
+## Push Notification Security
+- [x] VAPID key rotation capability
+- [x] Secure key generation using cryptography library
+- [x] Proper error handling for expired subscriptions
+- [ ] Consider adding payload encryption
+
+## General Security
+- [x] Type hints for better code safety
+- [x] Error logging implemented
+- [ ] Consider adding request logging
+- [ ] Consider adding CORS protection
+- [ ] Consider adding CSP headers 

models.py

@@ -1,25 +1,92 @@
+"""
+Database Models for Web Push Notification Service
+
+This module defines the SQLAlchemy models for managing web push notifications,
+including VAPID keys, user subscriptions, and application settings.
+
+Author: Matthew Raymer
+Created: 2025
+"""
+
 from flask_sqlalchemy import SQLAlchemy
+from typing import List, Optional
 
 db = SQLAlchemy()
 
 class VAPIDKey(db.Model):
+    """
+    Stores VAPID (Voluntary Application Server Identification) keys for
+    web push authentication.
+    
+    VAPID keys are used to identify the application server to push services
+    and establish a trust relationship for sending notifications.
+    
+    Attributes:
+        id (int): Primary key identifier
+        public_key (str): Base64-encoded public VAPID key (max 255 chars)
+        private_key (str): Base64-encoded private VAPID key (max 255 chars)
+        subscriptions (List[Subscription]): Related push notification subscriptions
+    """
+    
     id = db.Column(db.Integer, primary_key=True)
     public_key = db.Column(db.String(255), nullable=False)
     private_key = db.Column(db.String(255), nullable=False)
     subscriptions = db.relationship('Subscription', backref='vapid_key', lazy=True)
 
 class Settings(db.Model):
+    """
+    Application settings and state management for notification processing.
+    
+    Tracks the execution state of notification jobs to prevent duplicate
+    processing and maintain notification history.
+    
+    Attributes:
+        id (int): Primary key identifier
+        prev_notify_end_time (str): ISO 8601 timestamp of last completed notification run
+        running_notify_end_time (Optional[str]): ISO 8601 timestamp of currently running
+            notification job, or None if no job is running
+    """
+    
     id = db.Column(db.Integer, primary_key=True)
     prev_notify_end_time = db.Column(db.String(29), nullable=False)
     running_notify_end_time = db.Column(db.String(29), nullable=True)
 
 class Subscription(db.Model):
+    """
+    User subscription details for web push notifications.
+    
+    Stores the necessary information to send push notifications to a specific
+    browser/device, including encryption keys and notification preferences.
+    
+    Attributes:
+        id (int): Primary key identifier
+        auth (str): Authentication secret for push encryption (max 255 chars)
+        created_date (Optional[str]): ISO 8601 timestamp of subscription creation
+        endpoint (str): Push service URL for this subscription (max 500 chars)
+        message (Optional[str]): Custom message for direct notifications (max 100 chars)
+        notify_time (str): Daily notification time in "HH:MM" format (UTC)
+        notify_type (Optional[str]): Type of notification subscription
+            Valid values:
+            - 'DAILY_CHECK': Regular daily notifications
+            - 'DIRECT_NOTIFICATION': One-time direct notifications
+        p256dh (str): Client's public key for push encryption (max 255 chars)
+        vapid_key_id (int): Foreign key reference to associated VAPID key
+    
+    Relationships:
+        vapid_key: References the VAPID key used for this subscription
+    
+    Note:
+        The endpoint URL is unique per browser/device/user combination and
+        serves as the primary identifier for the subscription from the
+        push service's perspective.
+    """
+    
     id = db.Column(db.Integer, primary_key=True)
     auth = db.Column(db.String(255), nullable=False)
     created_date = db.Column(db.String(29), nullable=True)
     endpoint = db.Column(db.String(500), nullable=False)
     message = db.Column(db.String(100), nullable=True)
-    notify_time = db.Column(db.String(5), nullable=False) # HH:MM
+    notify_time = db.Column(db.String(5), nullable=False)  # HH:MM
-    notify_type = db.Column(db.String(32), nullable=True) # 'DAILY_CHECK', 'DIRECT_NOTIFICATION'
+    notify_type = db.Column(db.String(32), nullable=True)  # 'DAILY_CHECK', 'DIRECT_NOTIFICATION'
     p256dh = db.Column(db.String(255), nullable=False)
     vapid_key_id = db.Column(db.Integer, db.ForeignKey('vapid_key.id'), nullable=False)

requirements.txt

@@ -3,3 +3,4 @@ flask>=2.0.0
 flask_sqlalchemy
 pywebpush
 gunicorn
+structlog>=24.1.0