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/api/src/metadata_providers/mod.rs

@@ -0,0 +1,81 @@
+pub mod anilist;
+pub mod bedetheque;
+pub mod comicvine;
+pub mod google_books;
+pub mod open_library;
+
+use serde::{Deserialize, Serialize};
+
+/// Configuration passed to providers (API keys, etc.)
+#[derive(Debug, Clone, Default)]
+pub struct ProviderConfig {
+    pub api_key: Option<String>,
+    /// Preferred language for metadata results (ISO 639-1: "en", "fr", "es"). Defaults to "en".
+    pub language: String,
+}
+
+/// A candidate series returned by a provider search
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SeriesCandidate {
+    pub external_id: String,
+    pub title: String,
+    pub authors: Vec<String>,
+    pub description: Option<String>,
+    pub publishers: Vec<String>,
+    pub start_year: Option<i32>,
+    pub total_volumes: Option<i32>,
+    pub cover_url: Option<String>,
+    pub external_url: Option<String>,
+    pub confidence: f32,
+    pub metadata_json: serde_json::Value,
+}
+
+/// A candidate book within a series
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct BookCandidate {
+    pub external_book_id: String,
+    pub title: String,
+    pub volume_number: Option<i32>,
+    pub authors: Vec<String>,
+    pub isbn: Option<String>,
+    pub summary: Option<String>,
+    pub cover_url: Option<String>,
+    pub page_count: Option<i32>,
+    pub language: Option<String>,
+    pub publish_date: Option<String>,
+    pub metadata_json: serde_json::Value,
+}
+
+/// Trait that all metadata providers must implement
+pub trait MetadataProvider: Send + Sync {
+    #[allow(dead_code)]
+    fn name(&self) -> &str;
+
+    fn search_series(
+        &self,
+        query: &str,
+        config: &ProviderConfig,
+    ) -> std::pin::Pin<
+        Box<dyn std::future::Future<Output = Result<Vec<SeriesCandidate>, String>> + Send + '_>,
+    >;
+
+    fn get_series_books(
+        &self,
+        external_id: &str,
+        config: &ProviderConfig,
+    ) -> std::pin::Pin<
+        Box<dyn std::future::Future<Output = Result<Vec<BookCandidate>, String>> + Send + '_>,
+    >;
+}
+
+/// Factory function to get a provider by name
+pub fn get_provider(name: &str) -> Option<Box<dyn MetadataProvider>> {
+    match name {
+        "google_books" => Some(Box::new(google_books::GoogleBooksProvider)),
+        "open_library" => Some(Box::new(open_library::OpenLibraryProvider)),
+        "comicvine" => Some(Box::new(comicvine::ComicVineProvider)),
+        "anilist" => Some(Box::new(anilist::AniListProvider)),
+        "bedetheque" => Some(Box::new(bedetheque::BedethequeProvider)),
+        _ => None,
+    }
+}