Write face shards the way the rest of the catalog writes

The face store was the one part of the catalog still on SQLite's default
rollback journal at `synchronous = FULL`. The catalog itself runs WAL at
`NORMAL` (`schema::configure`) and so does the thumbnail store; nothing decided
this one should differ, it was simply never set.

Measured on this project's own filesystem, that is **21.3 ms per commit against
0.05 ms** — four hundred times. And an export commits four times per
photograph: the shard's transaction, then three separate autocommitting writes
to the index. Ten thousand images is on the order of fourteen minutes spent
doing nothing but waiting for fsync, before a byte goes to the server. That is
the "checking faces…" that appeared to hang.

So: WAL and `synchronous = NORMAL`, matching the rest, and the three index
writes fold into one transaction. `NORMAL` is the same trade the catalog makes —
a shard is derived data, and losing the last commit to a power cut costs one
image re-exported.

WAL brings an obligation with it, because **a shard is uploaded by reading its
file**: the newest commits live in a `-wal` sidecar that no upload sends, so
without a checkpoint the server would receive a database missing exactly the
faces just written, and a peer would adopt it and see nothing wrong. `checkpoint`
folds the logs back in, with `TRUNCATE` rather than the default passive mode,
which gives up when a reader holds the log and would leave the same gap while
reporting success.

Two tests: that the store is in WAL like everything else, and — the one that
matters — that a checkpointed shard copied *without* its `-wal` still holds
every face. That second one fails without the checkpoint, which is how it was
confirmed to be testing something.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/traceability.md

@@ -87,7 +87,7 @@ _None._
 | FR-EXP-9 | [`core/dr-decode/src/lib.rs:506`](../core/dr-decode/src/lib.rs#L506), [`core/dr-export/src/lib.rs:128`](../core/dr-export/src/lib.rs#L128), [`core/dr-export/src/lib.rs:1`](../core/dr-export/src/lib.rs#L1), [`core/dr-gpu/src/adjust.rs:1068`](../core/dr-gpu/src/adjust.rs#L1068), [`ui/dr-ui/src/develop.rs:2829`](../ui/dr-ui/src/develop.rs#L2829), [`ui/dr-ui/src/lib.rs:362`](../ui/dr-ui/src/lib.rs#L362) |
 | FR-NC-1 | [`core/dr-sync-nextcloud/src/auth.rs:132`](../core/dr-sync-nextcloud/src/auth.rs#L132), [`core/dr-sync-nextcloud/src/auth.rs:44`](../core/dr-sync-nextcloud/src/auth.rs#L44), [`core/dr-sync-nextcloud/src/session.rs:128`](../core/dr-sync-nextcloud/src/session.rs#L128), [`ui/dr-ui/src/launch.rs:256`](../ui/dr-ui/src/launch.rs#L256), [`ui/dr-ui/src/launch.rs:49`](../ui/dr-ui/src/launch.rs#L49), [`ui/dr-ui/src/launch_ui.rs:344`](../ui/dr-ui/src/launch_ui.rs#L344) |
 | FR-NC-10 | [`ui/dr-ui/src/export.rs:1`](../ui/dr-ui/src/export.rs#L1), [`ui/dr-ui/src/lib.rs:424`](../ui/dr-ui/src/lib.rs#L424), [`ui/dr-ui/src/library.rs:1049`](../ui/dr-ui/src/library.rs#L1049), [`ui/dr-ui/src/library.rs:1693`](../ui/dr-ui/src/library.rs#L1693), [`ui/dr-ui/src/library.rs:544`](../ui/dr-ui/src/library.rs#L544), [`ui/dr-ui/src/library.rs:846`](../ui/dr-ui/src/library.rs#L846), [`ui/dr-ui/src/library_ui.rs:1607`](../ui/dr-ui/src/library_ui.rs#L1607), [`ui/dr-ui/src/library_ui.rs:3365`](../ui/dr-ui/src/library_ui.rs#L3365), [`ui/dr-ui/src/library_ui.rs:499`](../ui/dr-ui/src/library_ui.rs#L499), [`ui/dr-ui/src/sidecar_cache.rs:1`](../ui/dr-ui/src/sidecar_cache.rs#L1) |
-| FR-NC-12 | [`core/dr-sync-nextcloud/src/lib.rs:34`](../core/dr-sync-nextcloud/src/lib.rs#L34), [`core/dr-sync-nextcloud/src/lib.rs:892`](../core/dr-sync-nextcloud/src/lib.rs#L892), [`core/dr-sync/src/lib.rs:157`](../core/dr-sync/src/lib.rs#L157), [`core/dr-sync/src/lib.rs:40`](../core/dr-sync/src/lib.rs#L40), [`core/dr-sync/src/reachability.rs:1`](../core/dr-sync/src/reachability.rs#L1), [`ui/dr-ui/src/remote.rs:1`](../ui/dr-ui/src/remote.rs#L1) |
+| FR-NC-12 | [`core/dr-sync-nextcloud/src/lib.rs:34`](../core/dr-sync-nextcloud/src/lib.rs#L34), [`core/dr-sync-nextcloud/src/lib.rs:904`](../core/dr-sync-nextcloud/src/lib.rs#L904), [`core/dr-sync/src/lib.rs:157`](../core/dr-sync/src/lib.rs#L157), [`core/dr-sync/src/lib.rs:40`](../core/dr-sync/src/lib.rs#L40), [`core/dr-sync/src/reachability.rs:1`](../core/dr-sync/src/reachability.rs#L1), [`ui/dr-ui/src/remote.rs:1`](../ui/dr-ui/src/remote.rs#L1) |
 | FR-NC-2 | [`core/dr-sync-nextcloud/src/session.rs:128`](../core/dr-sync-nextcloud/src/session.rs#L128), [`core/dr-sync-nextcloud/src/session.rs:34`](../core/dr-sync-nextcloud/src/session.rs#L34), [`platform/dr-plat/src/secrets.rs:82`](../platform/dr-plat/src/secrets.rs#L82) |
 | FR-NC-3 | [`core/dr-decode/src/locate.rs:1`](../core/dr-decode/src/locate.rs#L1), [`core/dr-decode/src/preview.rs:148`](../core/dr-decode/src/preview.rs#L148), [`core/dr-sync/src/capability.rs:41`](../core/dr-sync/src/capability.rs#L41), [`core/dr-thumbs/src/lib.rs:1`](../core/dr-thumbs/src/lib.rs#L1), [`ui/dr-ui/src/library.rs:1`](../ui/dr-ui/src/library.rs#L1), [`ui/dr-ui/src/library.rs:2724`](../ui/dr-ui/src/library.rs#L2724), [`ui/dr-ui/src/library.rs:3195`](../ui/dr-ui/src/library.rs#L3195), [`ui/dr-ui/src/library_ui.rs:1`](../ui/dr-ui/src/library_ui.rs#L1), [`ui/dr-ui/src/library_ui.rs:3627`](../ui/dr-ui/src/library_ui.rs#L3627), [`ui/dr-ui/src/library_ui.rs:4593`](../ui/dr-ui/src/library_ui.rs#L4593), [`ui/dr-ui/ui/app.slint:316`](../ui/dr-ui/ui/app.slint#L316), [`ui/dr-ui/ui/settings.slint:372`](../ui/dr-ui/ui/settings.slint#L372), [`ui/dr-ui/ui/settings.slint:72`](../ui/dr-ui/ui/settings.slint#L72) |
 | FR-NC-4 | [`core/dr-sync-nextcloud/src/propfind.rs:100`](../core/dr-sync-nextcloud/src/propfind.rs#L100), [`core/dr-sync-nextcloud/src/propfind.rs:51`](../core/dr-sync-nextcloud/src/propfind.rs#L51), [`core/dr-sync/src/capability.rs:6`](../core/dr-sync/src/capability.rs#L6), [`core/dr-sync/src/lib.rs:157`](../core/dr-sync/src/lib.rs#L157), [`core/dr-sync/src/scan.rs:93`](../core/dr-sync/src/scan.rs#L93), [`ui/dr-ui/src/launch.rs:49`](../ui/dr-ui/src/launch.rs#L49) |