docs: add comprehensive documentation files

Added documentation for API, architecture, configuration, database,
development guide, testing, and navigation. Includes helper scripts,
diagrams, and guides for NFO files and migration.

Docs/diagrams/download-flow.mmd

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+%%{init: {'theme': 'base'}}%%
+sequenceDiagram
+    participant Client
+    participant FastAPI
+    participant AuthMiddleware
+    participant DownloadService
+    participant ProgressService
+    participant WebSocketService
+    participant SeriesApp
+    participant Database
+
+    Note over Client,Database: Download Flow
+
+    %% Add to queue
+    Client->>FastAPI: POST /api/queue/add
+    FastAPI->>AuthMiddleware: Validate JWT
+    AuthMiddleware-->>FastAPI: OK
+    FastAPI->>DownloadService: add_to_queue()
+    DownloadService->>Database: save_item()
+    Database-->>DownloadService: item_id
+    DownloadService-->>FastAPI: [item_ids]
+    FastAPI-->>Client: 201 Created
+
+    %% Start queue
+    Client->>FastAPI: POST /api/queue/start
+    FastAPI->>AuthMiddleware: Validate JWT
+    AuthMiddleware-->>FastAPI: OK
+    FastAPI->>DownloadService: start_queue_processing()
+    
+    loop For each pending item
+        DownloadService->>SeriesApp: download_episode()
+        
+        loop Progress updates
+            SeriesApp->>ProgressService: emit("progress_updated")
+            ProgressService->>WebSocketService: broadcast_to_room()
+            WebSocketService-->>Client: WebSocket message
+        end
+        
+        SeriesApp-->>DownloadService: completed
+        DownloadService->>Database: update_status()
+    end
+    
+    DownloadService-->>FastAPI: OK
+    FastAPI-->>Client: 200 OK