Fix pipeline order, add selective metadata stripping, rename case/regex

- Move watermark step after compress in processing pipeline to match
  design doc order (resize, adjustments, convert, compress, metadata,
  watermark, rename)
- Implement selective EXIF metadata stripping for Privacy and Custom
  modes using little_exif tag filtering (GPS, camera, software,
  timestamps, copyright categories)
- Add case conversion support to rename (none/lower/upper/title)
- Add regex find-and-replace on original filenames
- Wire case and regex controls in rename step UI to JobConfig
- Add regex crate dependency to pixstrip-core

pixstrip-core/Cargo.toml

@@ -21,6 +21,7 @@ imageproc = "0.25"
 ab_glyph = "0.2"
 dirs = "6"
 notify = "7"
+regex = "1"
 
 [dev-dependencies]
 tempfile = "3"