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 {}

packages/image-engine/src/operations/compress.ts

@@ -87,5 +87,5 @@ async function compressToTargetSize(
       .toBuffer();
   }
 
-  return sharp(bestBuffer).toFormat(format, formatOpts(format, bestQuality));
+  return sharp(bestBuffer);
 }