feat: strip AI-provenance EXIF tags on removal (v0.5.6)

remove_ai_metadata now scrubs AI tags from the JPEG EXIF instead of passing
the block through wholesale. Closes the v0.5.5 follow-up: the xAI/Grok
Signature + UUID-Artist pair was detected but not removed.

- metadata._scrub_ai_exif(): deletes the xAI signature pair and any
  Software/Make/Artist/ImageDescription tag carrying an AI_GENERATOR_TOKENS
  token (so Ideogram's Make="Ideogram AI" is scrubbed too), keeping genuine
  camera/editor EXIF intact.
- Shared _is_xai_signature_pair / _exif_text helpers (module-level compiled
  regexes) are now the single source of truth, used by both xai_signature
  and _scrub_ai_exif.
- Tests: Grok signature stripped on JPEG output, Ideogram Make stripped,
  real-camera Make ("Apple") preserved. 325 passing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

uv.lock

@@ -2150,7 +2150,7 @@ wheels = [
 
 [[package]]
 name = "remove-ai-watermarks"
-version = "0.5.5"
+version = "0.5.6"
 source = { editable = "." }
 dependencies = [
     { name = "click" },