feat: add batch metadata jobs, series filters, and translate backoffice to French

- Add metadata_batch job type with background processing via tokio::spawn
- Auto-apply metadata only when single result at 100% confidence
- Support primary + fallback provider per library, "none" to opt out
- Add batch report/results API endpoints and job detail UI
- Add series_status and has_missing filters to both series listing pages
- Add GET /series/statuses endpoint for dynamic filter options
- Normalize series_metadata status values (migration 0036)
- Hide ComicVine provider tab when no API key configured
- Translate entire backoffice UI from English to French

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

infra/migrations/0034_add_metadata_batch.sql

@@ -0,0 +1,22 @@
+-- Add fallback_metadata_provider to libraries
+ALTER TABLE libraries ADD COLUMN fallback_metadata_provider TEXT;
+
+-- Table to store batch metadata job results (one row per series per job)
+CREATE TABLE metadata_batch_results (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    job_id UUID NOT NULL REFERENCES index_jobs(id) ON DELETE CASCADE,
+    library_id UUID NOT NULL REFERENCES libraries(id) ON DELETE CASCADE,
+    series_name TEXT NOT NULL,
+    status TEXT NOT NULL,  -- 'auto_matched', 'pending_review', 'no_results', 'too_many_results', 'low_confidence', 'already_linked', 'error'
+    provider_used TEXT,
+    fallback_used BOOLEAN NOT NULL DEFAULT FALSE,
+    candidates_count INTEGER NOT NULL DEFAULT 0,
+    best_confidence REAL,
+    best_candidate_json JSONB,
+    link_id UUID REFERENCES external_metadata_links(id) ON DELETE SET NULL,
+    error_message TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_mbr_job_id ON metadata_batch_results(job_id);
+CREATE INDEX idx_mbr_status ON metadata_batch_results(status);