fix(build): stop a stale or missing keepsake viewer from shipping silently

Three ways the compiled-in viewer could be wrong, none of which anything would
have reported. Found by mutation-testing the guard added below — it failed
when it should have passed, and the reason was the second bullet.

* `include_dir!` registers NO rebuild dependency. Run `npm run build` in
  frontend/export-viewer, then `cargo build`, and cargo sees no source change
  and reuses the cached binary — carrying the PREVIOUS index.html. The file on
  disk and the file in the binary disagree, git is clean, every check passes,
  and Memories.zip ships a stale viewer. Confirmed empirically: after replacing
  the artifact the compiled-in copy did not change until a source file was
  touched. A build.rs now declares `rerun-if-changed` for
  `static/export-viewer` AND `migrations` — sqlx::migrate!() embeds its
  directory the same way, and there the stale snapshot is worse still: the
  binary boots against a database that already ran a newer migration and
  crash-loops with VersionMissing.

* `emptyOutDir: true` deleted the committed artifact BEFORE generating. That
  was safe while the build could not fail; it no longer is, because
  `inlineThemeFonts` now calls `this.error` on a keepsake that is not
  self-contained. A failed build left the directory empty — and include_dir!
  over an empty directory compiles fine, while `write_viewer_with_data`
  iterates zero files and returns Ok. The result is a valid archive with every
  photo and no viewer. The output is one overwritten file, so nothing
  accumulates without the wipe.

* Nothing asserted the viewer was there at all. Now asserted at the point of
  use (bail rather than write a viewer-less keepsake) and in a test that checks
  presence, plausible size, and that no `url(/...)` survived inlining — the
  three ways it can be present but useless.

The Dockerfile copies build.rs with the sources rather than with Cargo.toml, so
the dependency-cache layer stays byte-identical and the dummy build does not
run it.

backend/Dockerfile

@@ -13,6 +13,13 @@ RUN mkdir src && echo "fn main(){}" > src/main.rs && \
 COPY src ./src
 COPY static ./static
 COPY migrations ./migrations
+# Copied WITH the sources, not with Cargo.toml above: cargo auto-detects `build.rs` by presence, so
+# putting it in the dependency-cache layer would make the dummy build run it too and invalidate a
+# layer that is otherwise stable. Copied at all because without it the image builds a subtly
+# DIFFERENT package from the one developers build — no build script, hence none of the
+# rerun-if-changed tracking for `static/export-viewer` and `migrations`. Harmless here (every image
+# build is clean, so there is no stale cache to reuse) and confusing everywhere else.
+COPY build.rs ./
 RUN touch src/main.rs && cargo build --release
 
 # --- Runtime stage ---