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.

docs/architecture/02-svelte-frontend.md

@@ -314,57 +314,22 @@ pub struct PlaybackModeManager {
 
 **Location**: `src-tauri/src/storage/db_service.rs`
 
-Async database interface wrapping synchronous `rusqlite` to prevent blocking the Tokio runtime:
+Async database interface over `rusqlite`. `RusqliteService` owns the
+database: writes run as jobs on one writer thread, reads on a pool of read-only
+WAL connections, so a read never waits for a write. Callers only see the
+trait (`execute`, `insert`, `execute_detached`, `query_one` / `query_optional` /
+`query_many`, `transaction`, `transaction_without_foreign_keys`) and build
+queries with `Query` + `QueryParam`, which keeps values out of the SQL string.
 
 ```rust
-#[async_trait]
-pub trait DatabaseService: Send + Sync {
-    async fn execute(&self, query: Query) -> Result<usize, DatabaseError>;
-    async fn execute_batch(&self, queries: Vec<Query>) -> Result<(), DatabaseError>;
-    async fn query_one<T, F>(&self, query: Query, mapper: F) -> Result<T, DatabaseError>
-        where F: FnOnce(&Row) -> Result<T> + Send + 'static;
-    async fn query_optional<T, F>(&self, query: Query, mapper: F) -> Result<Option<T>, DatabaseError>
-        where F: FnOnce(&Row) -> Result<T> + Send + 'static;
-    async fn query_many<T, F>(&self, query: Query, mapper: F) -> Result<Vec<T>, DatabaseError>
-        where F: Fn(&Row) -> Result<T> + Send + 'static;
-    async fn transaction<F, T>(&self, f: F) -> Result<T, DatabaseError>
-        where F: FnOnce(Transaction) -> Result<T> + Send + 'static;
-}
-
-pub struct RusqliteService {
-    connection: Arc<Mutex<Connection>>,
-}
-
-impl DatabaseService for RusqliteService {
-    async fn execute(&self, query: Query) -> Result<usize, DatabaseError> {
-        let conn = self.connection.clone();
-        tokio::task::spawn_blocking(move || {
-            // Execute query on blocking thread pool
-        }).await?
-    }
-    // ... other methods use spawn_blocking
-}
-```
-
-**Key Benefits:**
-- **No Freezing**: All blocking DB ops run in thread pool via `spawn_blocking`
-- **Type Safety**: `QueryParam` enum prevents SQL injection
-- **Future Proof**: Easy to swap to native async DB (tokio-rusqlite)
-- **Testable**: Can mock DatabaseService for tests
-
-**Usage Pattern:**
-```rust
-// Before (blocking - causes UI freeze)
-let conn = database.connection();
-let conn = conn.lock().unwrap();  // BLOCKS
-conn.query_row(...)  // BLOCKS
-
-// After (async - no freezing)
-let db_service = database.service();
+let db_service = database.service(); // cheap clone of the shared owner
 let query = Query::with_params("SELECT ...", vec![...]);
-db_service.query_one(query, |row| {...}).await  // spawn_blocking internally
+db_service.query_one(query, |row| {...}).await // runs on a reader
 ```
 
+Ownership model, invariants and the reasons for them:
+[08-database-design.md → Connection ownership](08-database-design.md#connection-ownership).
+
 ## Component Hierarchy
 
 ```mermaid