docs: complete API keys documentation in README.md

Documentation Enhancement:
- Add missing TVDB_API_KEY to all relevant sections in README.md
- Create comprehensive API keys reference table with purposes and sources
- Add centralized API Keys Configuration section with clear examples
- Document how to resolve "TVDB API key not configured" warnings
- Include direct links to obtain API keys from each service

API Keys Covered:
- TMDB_API_KEY - Movie release dates and metadata fallbacks
- TVDB_API_KEY - TV show metadata and Emby compatibility (was missing!)
- RADARR_API_KEY - Movie import history and database access
- SONARR_API_KEY - TV episode import history

This resolves user questions about the TVDB API key warning and provides
complete documentation for all external API integrations.

README.md

@@ -135,6 +135,7 @@ RADARR_DB_PASSWORD=your_actual_password
 
 # API keys
 TMDB_API_KEY=your_tmdb_api_key
+TVDB_API_KEY=your_tvdb_api_key
 RADARR_API_KEY=your_radarr_api_key
 SONARR_API_KEY=your_sonarr_api_key
 ```
@@ -480,6 +481,49 @@ ALLOW_FILE_DATE_FALLBACK=false
 
 ---
 
+## 🔑 API Keys Configuration
+
+NFOGuard integrates with multiple external APIs to provide enhanced metadata and release date information.
+
+### Required API Keys
+
+| Service | Environment Variable | Purpose | How to Get |
+|---------|---------------------|---------|------------|
+| **TMDB** | `TMDB_API_KEY` | Movie release dates, metadata fallbacks | Free at [themoviedb.org](https://www.themoviedb.org/settings/api) |
+| **TVDB** | `TVDB_API_KEY` | TV show metadata, Emby compatibility | Free at [thetvdb.com](https://thetvdb.com/api-information) |
+| **Radarr** | `RADARR_API_KEY` | Movie import history, database access | Found in Radarr Settings → General |
+| **Sonarr** | `SONARR_API_KEY` | TV episode import history | Found in Sonarr Settings → General |
+
+### Configuration Example
+
+Add to your `.env.secrets` file:
+
+```bash
+# External API keys for enhanced metadata
+TMDB_API_KEY=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+TVDB_API_KEY=your-tvdb-api-key-here
+
+# *arr application API keys  
+RADARR_API_KEY=a1b2c3d4e5f6...
+SONARR_API_KEY=f6e5d4c3b2a1...
+```
+
+### API Key Warnings
+
+If you see these warnings in logs, add the corresponding API key:
+
+```bash
+[WARNING] TMDB API key not configured - release date fallbacks disabled
+[WARNING] TVDB API key not configured, skipping TVDB ID lookup
+```
+
+**Note**: TMDB and TVDB keys are **optional** but highly recommended for:
+- ✅ Better release date accuracy
+- ✅ Enhanced TV show metadata  
+- ✅ Improved Emby/Plex compatibility
+
+---
+
 ## 📖 Example Workflow
 You add The Blacklist in Sonarr.