fix(images): pick the decoder from the bytes, not the extension

`image::open` chooses the decoder by extension, so a PNG saved as .jpg
(Android screenshots, chat exports) failed thumbnails, previews,
perceptual hashes, insights and face crops with "Illegal start bytes:
8950". New `file_types::open_image` sniffs the header and falls back to
the extension when it's unknown; every decode site uses it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/perceptual_hash.rs

@@ -30,7 +30,7 @@ pub struct PerceptualIdentity {
 /// callers should treat that as "no perceptual signal available" and
 /// proceed with exact-match dedup only.
 pub fn compute(path: &Path) -> Option<PerceptualIdentity> {
-    let img = image::open(path).ok()?;
+    let img = crate::file_types::open_image(path).ok()?;
 
     // 8x8 = 64 bits, the standard size for pHash/dHash. Larger sizes
     // give more discriminative power but no longer fit in i64 and the