Add dr-thumbs: a sharded, syncable thumbnail store

A thumbnail is the one derived artefact worth sending over the wire: it costs
a range fetch plus a decode to produce and is identical for every client
looking at the same file. A second device that downloads a shard gets a full
grid without fetching a byte of RAW.

Sharded at 25 MB, filled sequentially. The cap is about sync granularity, not
SQLite's limits — one growing database means every client re-downloads it
whenever a single thumbnail is added, whereas with sequential fill only the
newest shard is ever dirty and sealed shards are safe to cache forever.

Stored JPEG-encoded rather than as raw RGBA: a 256px RGBA buffer is ~256 KB
against ~20 KB encoded, and that 13x is transfer cost on every client.

Keyed on Nextcloud's oc:fileid, stable across server-side rename and move.

Assisted-by: LLM

Cargo.lock

@@ -1262,6 +1262,7 @@ dependencies = [
  "dr-types",
  "log",
  "rusqlite",
+ "serde_json",
  "thiserror 2.0.20",
 ]
 
@@ -1351,10 +1352,23 @@ dependencies = [
  "url",
 ]
 
+[[package]]
+name = "dr-thumbs"
+version = "0.1.0"
+dependencies = [
+ "dr-types",
+ "jpeg-encoder",
+ "log",
+ "rusqlite",
+ "thiserror 2.0.20",
+ "zune-jpeg 0.4.21",
+]
+
 [[package]]
 name = "dr-types"
 version = "0.1.0"
 dependencies = [
+ "serde",
  "thiserror 2.0.20",
 ]
 
@@ -1363,17 +1377,21 @@ name = "dr-ui"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "dr-catalog",
  "dr-decode",
  "dr-gpu",
  "dr-pipeline",
  "dr-plat",
  "dr-sync",
  "dr-sync-nextcloud",
+ "dr-thumbs",
  "dr-types",
  "log",
  "pollster",
  "reqwest",
+ "rusqlite",
  "serde_json",
+ "serde_norway",
  "slint",
  "slint-build",
  "tokio",
@@ -3115,6 +3133,12 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "jpeg-encoder"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a0370574b86f7eca156b9f298392b5e69a23f8c86f3f865add60bbc2e79467a6"
+
 [[package]]
 name = "js-sys"
 version = "0.3.104"
@@ -5333,6 +5357,12 @@ dependencies = [
  "unicode-script",
 ]
 
+[[package]]
+name = "ryu"
+version = "1.0.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9774ba4a74de5f7b1c1451ed6cd5285a32eddb5cccb8cc655a4e50009e06477f"
+
 [[package]]
 name = "same-file"
 version = "1.0.6"
@@ -5473,6 +5503,19 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "serde_norway"
+version = "0.9.42"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e408f29489b5fd500fab51ff1484fc859bb655f32c671f307dcd733b72e8168c"
+dependencies = [
+ "indexmap",
+ "itoa",
+ "ryu",
+ "serde",
+ "unsafe-libyaml-norway",
+]
+
 [[package]]
 name = "serde_repr"
 version = "0.1.21"
@@ -6554,6 +6597,12 @@ version = "0.2.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ebc1c04c71510c7f702b52b7c350734c9ff1295c464a03335b00bb84fc54f853"
 
+[[package]]
+name = "unsafe-libyaml-norway"
+version = "0.2.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b39abd59bf32521c7f2301b52d05a6a2c975b6003521cbd0c6dc1582f0a22104"
+
 [[package]]
 name = "untrusted"
 version = "0.9.0"