perf(db): reads no longer wait behind writes; pages answer from cache

A series page took about a second to show its seasons on a phone, every
visit, although they were cached. Three things stacked up:

- One SQLite connection behind one mutex served the whole app, so every
  read queued behind every write. The database now has one owner: a
  writer thread for writes and a pool of read-only WAL connections for
  reads. synchronous = NORMAL and a busy timeout on every connection.
- The listing query built the set of every available item in the
  database before filtering to the parent (~80 ms on a desktop for a
  100k-item cache), then fetched user data one row at a time. It now
  checks availability per row, uses the hierarchy indexes (1.5 ms on
  the same benchmark) and batches the user-data lookup.
- A cache read that missed the 100 ms fast path was set aside until the
  server answered. It is now raced against the server; whichever answers
  first with content wins.

On the Fairphone, Frasier's season and episode lists now come from
cache in 34-133 ms (was 600-1030 ms waiting on the server).

Fixes found on the way, each with a test that failed first:
- sync_queue_mutation could return another mutation's row id: the id
  came from a second trip to the shared connection. insert() reads it in
  the same job.
- save_to_cache switched foreign keys off on the shared connection
  across its awaits, so concurrent writes ran unchecked. The toggle now
  lives inside one writer job, and a page is one transaction instead of
  one commit per row.

Also: thumbnail LRU touches no longer block the lookup; unused
tokio-rusqlite dropped. Design and invariants in
docs/architecture/08-database-design.md (Connection ownership, Listing
query shape) and 03-data-flow.md.

src-tauri/Cargo.lock

@@ -2315,7 +2315,6 @@ dependencies = [
  "tempfile",
  "tiny_http",
  "tokio",
- "tokio-rusqlite",
  "tokio-util",
  "urlencoding",
  "uuid",
@@ -5340,17 +5339,6 @@ dependencies = [
  "syn 2.0.112",
 ]
 
-[[package]]
-name = "tokio-rusqlite"
-version = "0.6.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b65501378eb676f400c57991f42cbd0986827ab5c5200c53f206d710fb32a945"
-dependencies = [
- "crossbeam-channel",
- "rusqlite",
- "tokio",
-]
-
 [[package]]
 name = "tokio-rustls"
 version = "0.26.4"