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/tests/operations_tests.rs

@@ -92,6 +92,9 @@ fn rename_config_simple_template() {
         counter_start: 1,
         counter_padding: 3,
         template: None,
+        case_mode: 0,
+        regex_find: String::new(),
+        regex_replace: String::new(),
     };
     let result = config.apply_simple("sunset", "jpg", 1);
     assert_eq!(result, "blog_sunset_001.jpg");
@@ -105,6 +108,9 @@ fn rename_config_with_suffix() {
         counter_start: 1,
         counter_padding: 2,
         template: None,
+        case_mode: 0,
+        regex_find: String::new(),
+        regex_replace: String::new(),
     };
     let result = config.apply_simple("photo", "webp", 5);
     assert_eq!(result, "photo_web_05.webp");