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/bg-effects.ts

@@ -132,7 +132,7 @@ export async function compositeOnColor(subjectBuffer: Buffer, hexColor: string):
       width: meta.width,
       height: meta.height,
       channels: 4,
-      background: { r, g, b, alpha: 255 },
+      background: { r, g, b, alpha: 1 },
     },
   })
     .composite([{ input: subjectBuffer, blend: "over" }])
@@ -223,7 +223,7 @@ export async function applyEffects(
     const g = parseInt(hex.substring(2, 4), 16);
     const b = parseInt(hex.substring(4, 6), 16);
     background = await sharp({
-      create: { width, height, channels: 4, background: { r, g, b, alpha: 255 } },
+      create: { width, height, channels: 4, background: { r, g, b, alpha: 1 } },
     })
       .png()
       .toBuffer();