feat: add external metadata sync system with multiple providers

Add a complete metadata synchronization system allowing users to search
and sync series/book metadata from external providers (Google Books,
Open Library, ComicVine, AniList, Bédéthèque). Each library can use a
different provider. Matching requires manual approval with detailed sync
reports showing what was updated or skipped (locked fields protection).

Key changes:
- DB migrations: external_metadata_links, external_book_metadata tables,
  library metadata_provider column, locked_fields, total_volumes, book
  metadata fields (summary, isbn, publish_date)
- Rust API: MetadataProvider trait + 5 provider implementations,
  7 metadata endpoints (search, match, approve, reject, links, missing,
  delete), sync report system, provider language preference support
- Backoffice: MetadataSearchModal, ProviderIcon, SafeHtml components,
  settings UI for provider/language config, enriched book detail page,
  edit forms with locked fields support, API proxy routes
- OpenAPI/Swagger documentation for all new endpoints and schemas

Closes #3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/app/components/SafeHtml.tsx

@@ -0,0 +1,29 @@
+import sanitizeHtml from "sanitize-html";
+import React from "react";
+
+interface SafeHtmlProps {
+  html: string;
+  className?: string;
+  as?: "div" | "p" | "span";
+}
+
+const sanitizeOptions: sanitizeHtml.IOptions = {
+  allowedTags: [
+    "b", "i", "em", "strong", "a", "p", "br", "ul", "ol", "li",
+    "h1", "h2", "h3", "h4", "h5", "h6", "blockquote", "span",
+  ],
+  allowedAttributes: {
+    a: ["href", "target", "rel"],
+    span: ["class"],
+  },
+  transformTags: {
+    a: sanitizeHtml.simpleTransform("a", { target: "_blank", rel: "noopener noreferrer" }),
+  },
+};
+
+export function SafeHtml({ html, className, as: tag = "div" }: SafeHtmlProps) {
+  return React.createElement(tag, {
+    className,
+    dangerouslySetInnerHTML: { __html: sanitizeHtml(html, sanitizeOptions) },
+  });
+}