feat: remove startup NFO repair, update docs and tests

- Remove NFO repair scan step from ARCHITECTURE.md startup sequence
- Update CHANGELOG.md: rephrase perform_nfo_repair_scan as scheduled scan
- Add test verifying perform_nfo_repair_scan is NOT called in lifespan
- Keep existing folder scan wiring tests and unit tests intact
- NFO_GUIDE.md already correctly describes scheduled scan behavior

docs/NFO_GUIDE.md

@@ -246,7 +246,84 @@ NFO files are created in the anime directory:
 
 ---
 
-## 5. API Reference
+## 5. Folder Naming Convention
+
+### 5.1 Expected Format
+
+After the daily folder scan (when **Update on library scan** is enabled), Aniworld validates every series folder against its `tvshow.nfo` metadata. If the folder name does not match the expected convention, it is automatically renamed.
+
+**Format:**
+
+```
+{title} ({year})
+```
+
+**Examples:**
+
+| NFO `<title>` | NFO `<year>` | Expected Folder Name |
+|---------------|--------------|----------------------|
+| `Attack on Titan` | `2013` | `Attack on Titan (2013)` |
+| `One Piece` | `1999` | `One Piece (1999)` |
+| `Demon Slayer: Kimetsu no Yaiba` | `2019` | `Demon Slayer Kimetsu no Yaiba (2019)` |
+
+### 5.2 Sanitization Rules
+
+Illegal filesystem characters are removed or replaced to ensure cross-platform compatibility:
+
+- Removed: `< > : " / \ | ? *` and null bytes
+- Control characters stripped
+- Multiple spaces collapsed to one
+- Leading/trailing dots and whitespace trimmed
+- Maximum length: 200 characters (truncated at word boundary if possible)
+
+### 5.3 Skip Conditions
+
+A folder is **not** renamed when any of the following apply:
+
+- `tvshow.nfo` is missing `<title>` or `<year>` (or they are empty)
+- The series has an **active or pending download**
+- The target folder name already exists (duplicate)
+- The resulting path would exceed the OS path-length limit
+- The app lacks write permission to the anime directory
+
+All skipped and renamed actions are logged.
+
+---
+
+## 6. Poster Check
+
+### 6.1 Overview
+
+During the daily folder scan, Aniworld checks every series folder for a valid `poster.jpg`. If the file is missing or smaller than 1 KB, the application attempts to re-download it from the URL stored in the series' `tvshow.nfo` file.
+
+### 6.2 How It Works
+
+1. **Scan** — After folder renaming, the scan iterates over all series folders that contain a `tvshow.nfo`.
+2. **Validate** — For each folder, it checks whether `poster.jpg` exists and is at least 1 KB.
+3. **Parse NFO** — If the poster is missing or too small, the scan reads `tvshow.nfo` and looks for a `<thumb aspect="poster">` (or any `<thumb>`) URL.
+4. **Download** — If a URL is found, the poster is downloaded using `ImageDownloader` with a concurrency limit of 3 simultaneous downloads.
+5. **Validate Download** — The downloaded image is validated with PIL to ensure it is not corrupted.
+
+### 6.3 Skip Conditions
+
+A folder is **not** processed for poster download when any of the following apply:
+
+- `tvshow.nfo` does not exist in the folder.
+- `poster.jpg` already exists and is ≥ 1 KB.
+- No `<thumb>` URL is found in the NFO (the NFO may have been created before thumb tags were added).
+- The `nfo.download_poster` setting is `false` (poster checks are still performed, but downloads are skipped if the setting is disabled; see [CONFIGURATION.md](CONFIGURATION.md)).
+
+### 6.4 Logging
+
+Every poster check action is logged:
+
+- **INFO** — When a poster is successfully downloaded.
+- **WARNING** — When a download fails or no URL is found.
+- **ERROR** — When an unexpected exception occurs during download.
+
+---
+
+## 7. API Reference
 
 ### 5.1 Check NFO Status