Add comprehensive deletion detection for removed/unavailable content

Implements issue #335: improve detection of deleted/missing posts

## Changes

### New Deletion Detection System
- Created `deletion_detection.py` utility module with platform-specific
  indicators for Twitter, Facebook, Instagram, TikTok, YouTube, Reddit,
  VK, and Telegram
- Detects deletion via HTML content, page titles, error messages, and
  video metadata
- Stores detailed deletion context (indicator, source, platform) in
  metadata for investigators

### Integration Points
- **Antibot Extractor**: Checks HTML and page titles after page load;
  resolves TODO about detecting deleted videos
- **Generic Extractor**: Checks yt-dlp video data and error messages
  for deletion indicators
- **Twitter Dropin**: Enhanced detection when user/created_at fields
  are missing

### Test Coverage
- Comprehensive test suite covering all platforms
- Tests for HTML, title, error message, and metadata detection
- Validates that normal content is not falsely flagged

## Impact for Conflict Documentation

This fix is critical for evidence preservation in war-torn regions:
- Investigators can now document that evidence existed but was deleted
- Prevents wasted archival attempts on deleted content
- Tracks patterns of content removal
- Preserves metadata about what was deleted and when

Twitter example: Detects "Hmm...this page doesn't exist. Try searching
for something else" and flags content as deleted_or_unavailable.

src/auto_archiver/modules/generic_extractor/twitter.py

@@ -7,6 +7,7 @@ from slugify import slugify
 from auto_archiver.core.metadata import Metadata, Media
 from auto_archiver.utils import url as UrlUtil, get_datetime_from_str
 from auto_archiver.core.extractor import Extractor
+from auto_archiver.utils.deletion_detection import detect_deletion, flag_as_deleted
 from auto_archiver.modules.generic_extractor.dropin import GenericDropin, InfoExtractor
 
 
@@ -37,7 +38,17 @@ class Twitter(GenericDropin):
         result = Metadata()
         try:
             if not tweet.get("user") or not tweet.get("created_at"):
-                raise ValueError("Error retreiving post. Are you sure it exists?")
+                # Check for deletion indicators
+                deletion_info = detect_deletion(
+                    video_data=tweet,
+                    url=url,
+                    error_message="Missing user or created_at fields"
+                )
+                if deletion_info:
+                    flag_as_deleted(result, deletion_info)
+                    return result
+
+                raise ValueError("Error retrieving post. Are you sure it exists?")
             timestamp = get_datetime_from_str(tweet["created_at"], "%a %b %d %H:%M:%S %z %Y")
         except (ValueError, KeyError) as ex:
             logger.warning(f"Unable to parse tweet: {str(ex)}\nRetreived tweet data: {tweet}")