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

core/dr-thumbs/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "dr-thumbs"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+
+[dependencies]
+dr-types.workspace = true
+rusqlite.workspace = true
+jpeg-encoder.workspace = true
+zune-jpeg.workspace = true
+thiserror.workspace = true
+log.workspace = true