fix: resolve 14 security, correctness, and robustness issues found during QA sweep

Security fixes:
- Add auth + ownership check to thumbnail endpoint (was unauthenticated)
- Validate ExifTool fieldsToRemove against safe tag name pattern
- Add SVG sanitization to pipeline execute and batch endpoints
- Replace basename() with sanitizeFilename() in 16 tool routes
- Escape SQL LIKE wildcards in file search to prevent pattern injection
- Improve settings HTML tag validation pattern

Bug fixes:
- Skip autoOrient for SVG inputs in pipeline (prevents misinterpretation)
- Remove double-encode in compress targetSize (was degrading quality)
- Fix bg-effects alpha value from 255 to 1.0 (Sharp expects float)
- Guard download stream error handler against headers-already-sent race
- Use O_EXCL atomic file creation for install lock (fixes TOCTOU race)
- Truncate collage file array to template image count

UX fixes:
- Accept empty JSON bodies on POST endpoints (install/uninstall)
- Custom JSON content type parser that treats empty body as {}

apps/api/src/lib/exiftool.ts

@@ -234,10 +234,12 @@ export function buildTagArgs(settings: EditMetadataSettings): string[] {
   if (settings.iptcState) args.push(`-IPTC:Province-State=${settings.iptcState}`);
   if (settings.iptcCountry) args.push(`-IPTC:Country-PrimaryLocationName=${settings.iptcCountry}`);
 
-  // Field removal
+  // Field removal -- only allow safe EXIF/IPTC/XMP tag names (alphanumeric, colon, hyphen)
   if (settings.fieldsToRemove && settings.fieldsToRemove.length > 0) {
     for (const field of settings.fieldsToRemove) {
-      args.push(`-${field}=`);
+      if (/^[A-Za-z0-9:_-]+$/.test(field)) {
+        args.push(`-${field}=`);
+      }
     }
   }