feat(nfo): add minimal NFO fallback when TMDB fails

- Add create_minimal_nfo() method to NFOService for fallback when TMDB lookup fails
- Update API endpoints (single and batch) to use minimal NFO fallback on TMDBAPIError
- Document fallback behavior in NFO_GUIDE.md section 3.6
- Add unit tests for minimal NFO creation (11 tests passing)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/NFO_GUIDE.md

@@ -171,6 +171,35 @@ Response:
 }
 ```
 
+### 3.6 Fallback Behavior When TMDB is Unavailable
+
+When TMDB lookup fails (network issues, API errors, or no match found), the system creates a **minimal NFO** to ensure the series is still tracked. This behavior applies to:
+
+- Manual NFO creation via API
+- Batch NFO creation operations
+- Automatic NFO creation during downloads
+
+**What a minimal NFO contains:**
+
+```xml
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<tvshow>
+    <title>Series Name</title>
+    <year>2024</year>
+    <plot>No metadata available for Series Name. TMDB lookup failed.</plot>
+</tvshow>
+```
+
+**Limitations of minimal NFOs:**
+- No poster, logo, or fanart images
+- No rating, genre, or studio information
+- No TMDB or other provider IDs
+- May not display correctly in some media servers
+
+**To upgrade a minimal NFO:**
+1. Use the Update endpoint (`PUT /api/nfo/{serie_id}/update`) when TMDB is available
+2. Or delete the NFO and recreate it with full metadata
+
 ---
 
 ## 4. File Structure