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/README.md

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+# Architecture Diagrams
+
+This directory contains architecture diagram source files for the Aniworld documentation.
+
+## Diagrams
+
+### System Architecture (Mermaid)
+
+See [system-architecture.mmd](system-architecture.mmd) for the system overview diagram.
+
+### Rendering
+
+Diagrams can be rendered using:
+
+-   Mermaid Live Editor: https://mermaid.live/
+-   VS Code Mermaid extension
+-   GitHub/GitLab native Mermaid support
+
+## Formats
+
+-   `.mmd` - Mermaid diagram source files
+-   `.svg` - Exported vector graphics (add when needed)
+-   `.png` - Exported raster graphics (add when needed)

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

Docs/diagrams/system-architecture.mmd

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+%%{init: {'theme': 'base', 'themeVariables': { 'primaryColor': '#4a90d9'}}}%%
+flowchart TB
+    subgraph Clients["Client Layer"]
+        Browser["Web Browser<br/>(HTML/CSS/JS)"]
+        CLI["CLI Client<br/>(Main.py)"]
+    end
+
+    subgraph Server["Server Layer (FastAPI)"]
+        direction TB
+        Middleware["Middleware<br/>Auth, Rate Limit, Error Handler"]
+        
+        subgraph API["API Routers"]
+            AuthAPI["/api/auth"]
+            AnimeAPI["/api/anime"]
+            QueueAPI["/api/queue"]
+            ConfigAPI["/api/config"]
+            SchedulerAPI["/api/scheduler"]
+            HealthAPI["/health"]
+            WebSocketAPI["/ws"]
+        end
+        
+        subgraph Services["Services"]
+            AuthService["AuthService"]
+            AnimeService["AnimeService"]
+            DownloadService["DownloadService"]
+            ConfigService["ConfigService"]
+            ProgressService["ProgressService"]
+            WebSocketService["WebSocketService"]
+        end
+    end
+
+    subgraph Core["Core Layer"]
+        SeriesApp["SeriesApp"]
+        SeriesCache["SeriesCache<br/>(In-Memory)"]
+        SerieScanner["SerieScanner"]
+        SerieList["SerieList"]
+    end
+
+    subgraph Data["Data Layer"]
+        SQLite[("SQLite<br/>aniworld.db")]
+        ConfigJSON[(config.json)]
+        FileSystem[(File System<br/>Anime Episodes)]
+        LegacyFiles[("Legacy Files<br/>key/data<br/>(Deprecated)")]
+    end
+
+    subgraph External["External"]
+        Provider["Anime Provider<br/>(aniworld.to)"]
+    end
+
+    %% Client connections
+    Browser -->|HTTP/WebSocket| Middleware
+    CLI -->|Direct| SeriesApp
+
+    %% Middleware to API
+    Middleware --> API
+
+    %% API to Services
+    AuthAPI --> AuthService
+    AnimeAPI --> AnimeService
+    QueueAPI --> DownloadService
+    ConfigAPI --> ConfigService
+    SchedulerAPI --> AnimeService
+    WebSocketAPI --> WebSocketService
+
+    %% Services to Core
+    AnimeService --> SeriesApp
+    DownloadService --> SeriesApp
+
+    %% Services to Data
+    AuthService --> ConfigJSON
+    ConfigService --> ConfigJSON
+    DownloadService --> SQLite
+    AnimeService --> SQLite
+
+    %% Core to Data
+    SeriesApp --> SeriesCache
+    SeriesCache -.->|Cached Series| SQLite
+    SeriesApp --> SerieScanner
+    SeriesApp --> SerieList
+    SerieScanner -->|Scan Episodes| FileSystem
+    SerieScanner -->|Detect Series| SQLite
+    SerieScanner -->|Migrate Legacy| LegacyFiles
+    SerieScanner --> Provider
+
+    %% Event flow
+    ProgressService -.->|Events| WebSocketService
+    DownloadService -.->|Progress| ProgressService
+    WebSocketService -.->|Broadcast| Browser