feat(metadata): write-to-file — embedded EPUB OPF + Calibre sidecars

Phase 5a of the metadata-writer feature: the book editor gains a
"Write to file" action (admin) that persists the item's CURRENT
database metadata into the artifact on disk, so the library stays
portable to Calibre and other apps — the thing Save alone can't do.

Dispatch (services/metadata_writer.go): EPUB/KEPUB gets its embedded
OPF spliced in place (fresh dc:* block generated inside the ORIGINAL
metadata open tag; manifest/spine/guide untouched; every other zip
entry copied byte-identical in original order so mimetype stays first
and STORED; original dc:identifiers preserved — the package's
unique-identifier must keep resolving). Everything else takes a
Calibre-style sidecar: metadata.opf when the book is ALONE in its
directory (Calibre's own convention — round-trips through Calibre
too), <basename>.opf when siblings share the folder, so one book's
sidecar can never poison dir-mates' scans (the scanner now reads both,
basename first).

Safety contract: temp file + atomic rename — the original is never
truncated on failure; the generated OPF is unit-tested to ROUND-TRIP
through the scanner's own parser (write side and read side can never
disagree); metadata_overrides are deliberately kept (file and DB agree
after the write; overrides still guard against stale sidecars). The
endpoint re-extracts inline (RescanMediaItem) and reports rescan
failure separately — a failed rescan doesn't undo the write. SHA-256
changes are inherent and documented for users (Kobo/KOReader re-key).

UI: the metadata editor modal carries the file path as a data
attribute; the button confirms before mutating, labels the target
(embedded vs sidecar) and toasts the outcome. Integration tests pin
the 403, missing-file error, EPUB embed + post-write rescan title, the
lone-book metadata.opf case, and the shared-dir <basename>.opf case
with no dir-scoped litter.

templates/book_detail_modals.templ

@@ -251,6 +251,7 @@ templ MetadataEditorModal(book handlers.MediaDetail) {
 		id="metadata-editor-modal"
 		class="hidden fixed inset-0 z-50 flex items-center justify-center overflow-y-auto p-4"
 		style="background-color: var(--surface-overlay);"
+		data-write-file-path={ book.FilePath }
 	>
 		<div
 			class="card w-full max-w-5xl my-8 flex flex-col max-h-[90vh]"
@@ -683,6 +684,18 @@ templ MetadataEditorModal(book handlers.MediaDetail) {
 						<path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z"></path>
 					</svg>
 				</button>
+				<button
+					@click="writeMetadataToFile()"
+					:disabled="writingFile"
+					class="btn btn-secondary"
+					:title="metadataWriteHint()"
+				>
+					<span x-show="!writingFile" class="inline-flex items-center gap-2">@Icon("save", "h-4 w-4")<span>Write to file</span></span>
+					<svg x-show="writingFile" class="animate-spin inline-block h-5 w-5" viewBox="0 0 24 24" fill="none">
+						<circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
+						<path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z"></path>
+					</svg>
+				</button>
 			</div>
 			<div class="flex space-x-3">
 				<button