feat: implement real IMAP message parsing with native CouchDB attachments

- Replace placeholder message generation with actual IMAP message fetching using go-message library
- Add per-account CouchDB databases for better organization and isolation
- Implement native CouchDB attachment storage with proper revision management
- Add command line argument parsing with --max-messages flag for controlling message processing limits
- Support both sync and archive modes with proper document synchronization
- Add comprehensive test environment with Podman containers (GreenMail IMAP server + CouchDB)
- Implement full MIME multipart parsing for proper body and attachment extraction
- Add TLS and plain IMAP connection support based on port configuration
- Update configuration system to support sync vs archive modes
- Create test scripts and sample data for development and testing

Key technical improvements:
- Real email envelope and header processing with go-imap v2 API
- MIME Content-Type and Content-Disposition parsing for attachment detection
- CouchDB document ID generation using mailbox_uid format for uniqueness
- Duplicate detection and prevention to avoid re-storing existing messages
- Proper error handling and connection management for IMAP operations

🤖 Generated with [Claude Code](https://claude.ai/code)

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

test/config-test.json

@@ -0,0 +1,55 @@
+{
+  "couchDb": {
+    "url": "http://localhost:5984",
+    "user": "admin",
+    "password": "password",
+    "database": "mail_backup_test"
+  },
+  "mailSources": [
+    {
+      "name": "Test User 1",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "testuser1",
+      "password": "password123",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["INBOX", "Sent"],
+        "exclude": []
+      },
+      "messageFilter": {}
+    },
+    {
+      "name": "Test Sync User", 
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "syncuser",
+      "password": "syncpass",
+      "mode": "sync",
+      "folderFilter": {
+        "include": ["INBOX"],
+        "exclude": []
+      },
+      "messageFilter": {}
+    },
+    {
+      "name": "Test Archive User",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost", 
+      "port": 3143,
+      "user": "archiveuser",
+      "password": "archivepass",
+      "mode": "archive",
+      "folderFilter": {
+        "include": [],
+        "exclude": ["Drafts"]
+      },
+      "messageFilter": {}
+    }
+  ]
+}