fix: block PDF uploads, fix PBM/PGM/PPM batch failures, add tests

- Added isImageFile() filter to all drop handlers (dropzone, collage,
  file-upload-area) so PDFs and non-image files are rejected on drop.
  Previously only the file picker's accept attribute filtered; drag-and-
  drop accepted anything.

- Added ppm, pgm, pbm to CLI_DECODED_FORMATS in file-validation.ts.
  These formats were missing, causing Sharp metadata checks to fail for
  some files during batch validation, which silently dropped them from
  results ("File not found in batch results").

- Added integration tests for PBM, PGM, PPM, TIFF, QOI, JP2, SVGZ
  single-file processing, plus a test confirming PDF is rejected.

apps/api/src/lib/file-validation.ts

@@ -172,6 +172,9 @@ const CLI_DECODED_FORMATS = new Set([
   "cur",
   "dpx",
   "fits",
+  "ppm",
+  "pgm",
+  "pbm",
 ]);
 
 /**