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/src/error.rs

@@ -0,0 +1,21 @@
+//! TRACES: NFR-ARCH-4
+//! Thumbnail store errors.
+//!
+//! A thumbnail is a cache entry, never authoritative — every failure here is
+//! recoverable by regenerating from the source, so nothing in this module is
+//! fatal to the app.
+
+/// Something went wrong reading or writing the store.
+#[derive(Debug, thiserror::Error)]
+pub enum ThumbError {
+    #[error("sqlite: {0}")]
+    Sqlite(#[from] rusqlite::Error),
+
+    /// The index names a shard whose file is absent — a partial sync, or a
+    /// manual deletion. The entry is treated as a miss and regenerated.
+    #[error("shard {0} is missing")]
+    MissingShard(u32),
+
+    #[error("io: {0}")]
+    Io(String),
+}