feat: add content_hash backfill + register every media file

Adds blake3 content hashing as the basis for derivative dedup
(thumbnails, HLS) across libraries. Computed inline by the watcher on
ingest and by a new `backfill_hashes` binary for historical rows.

Key changes:
- `content_hash` and `size_bytes` are now populated on new image_exif
  rows; a new ExifDao surface (`get_rows_missing_hash`,
  `backfill_content_hash`, `find_by_content_hash`) supports backfill and
  future hash-keyed lookups.
- The watcher now registers every image/video in image_exif, not just
  files with parseable EXIF. EXIF becomes optional enrichment; videos
  and other non-EXIF files still get a hashed row. This also makes
  DB-indexed sort/filter cover the full library.
- `/image` thumbnail serve dual-looks up hash-keyed path first, then
  falls back to the legacy mirrored layout.
- Upload flow accepts `?library=` query param + hashes uploaded files.
- Store_exif logs the underlying Diesel error on insert failure so
  constraint violations surface instead of hiding behind a generic
  InsertError.
- New migration normalizes rel_path separators to forward slash across
  all tables, deduplicating any rows that collide after normalization.
  Fixes spurious UNIQUE violations from mixed backslash/forward-slash
  paths on Windows ingest.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.lock

@@ -474,6 +474,12 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "arrayref"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "76a2e8124351fda1ef8aaaa3bbd7ebbcb486bbcd4225aca0aa0d84bb2db8fecb"
+
 [[package]]
 name = "arrayvec"
 version = "0.7.6"
@@ -572,6 +578,20 @@ version = "2.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6099cdc01846bc367c4e7dd630dc5966dccf36b652fae7a74e17b640411a91b2"
 
+[[package]]
+name = "blake3"
+version = "1.8.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4d2d5991425dfd0785aed03aedcf0b321d61975c9b5b3689c774a2610ae0b51e"
+dependencies = [
+ "arrayref",
+ "arrayvec",
+ "cc",
+ "cfg-if",
+ "constant_time_eq",
+ "cpufeatures 0.3.0",
+]
+
 [[package]]
 name = "block-buffer"
 version = "0.10.4"
@@ -766,6 +786,12 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b05b61dc5112cbb17e4b6cd61790d9845d13888356391624cbe7e41efeac1e75"
 
+[[package]]
+name = "constant_time_eq"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d52eff69cd5e647efe296129160853a42795992097e8af39800e1060caeea9b"
+
 [[package]]
 name = "convert_case"
 version = "0.4.0"
@@ -808,6 +834,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "cpufeatures"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8b2a41393f66f16b0823bb79094d54ac5fbd34ab292ddafb9a0456ac9f87d201"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "crc32fast"
 version = "1.5.0"
@@ -1810,6 +1845,7 @@ dependencies = [
  "anyhow",
  "base64",
  "bcrypt",
+ "blake3",
  "chrono",
  "clap",
  "diesel",
@@ -3365,7 +3401,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e3bf829a2d51ab4a5ddf1352d8470c140cadc8301b2ae1789db023f01cedd6ba"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "digest",
 ]