feat(fts): add search_vector column, GIN index, DB triggers, and FTS repository method (V34)

- V34 migration: adds search_vector tsvector column with GIN index
- BEFORE INSERT/UPDATE trigger on documents rebuilds vector from title (A),
  summary + transcription_blocks.text (B), sender/receiver names (C),
  tag names + location (D) using german FTS config
- AFTER triggers on transcription_blocks, document_receivers, document_tags
  touch the parent document row to re-fire the BEFORE UPDATE trigger
- DocumentRepository.findRankedIdsByFts() native query using websearch_to_tsquery
- DocumentFtsTest: 12 integration tests covering stemming, trigger sync,
  ranking, stop words, malformed input, receiver and tag search

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

backend/src/main/resources/db/migration/V34__add_fts_search_vector.sql

@@ -0,0 +1,74 @@
+-- ─── Full-Text Search: search_vector on documents ──────────────────────────────
+-- Adds a tsvector column that aggregates: title (A), summary + transcription
+-- block text (B), sender/receiver names (C), tag names + location (D).
+-- The column is maintained by DB triggers so the OCR pipeline (which writes
+-- transcription_blocks directly) stays in sync without JPA @PreUpdate hooks.
+
+-- 1. Column and GIN index
+ALTER TABLE documents ADD COLUMN search_vector tsvector;
+CREATE INDEX idx_documents_search ON documents USING GIN (search_vector);
+
+-- 2. Trigger function: rebuilds search_vector on documents INSERT or UPDATE.
+--    Runs BEFORE the write so NEW.search_vector is set inline.
+CREATE OR REPLACE FUNCTION fn_documents_fts_update() RETURNS trigger AS $$
+BEGIN
+  NEW.search_vector :=
+    setweight(to_tsvector('german', coalesce(NEW.title,   '')), 'A') ||
+    setweight(to_tsvector('german', coalesce(NEW.summary, '')), 'B') ||
+    setweight(to_tsvector('german', coalesce((
+      SELECT string_agg(tb.text, ' ') FILTER (WHERE tb.text IS NOT NULL)
+      FROM   transcription_blocks tb
+      WHERE  tb.document_id = NEW.id
+    ), '')), 'B') ||
+    setweight(to_tsvector('german', coalesce((
+      SELECT coalesce(p.first_name, '') || ' ' || p.last_name
+      FROM   persons p
+      WHERE  p.id = NEW.sender_id
+    ), '')), 'C') ||
+    setweight(to_tsvector('german', coalesce((
+      SELECT string_agg(coalesce(p.first_name, '') || ' ' || p.last_name, ' ')
+      FROM   document_receivers dr
+      JOIN   persons p ON p.id = dr.person_id
+      WHERE  dr.document_id = NEW.id
+    ), '')), 'C') ||
+    setweight(to_tsvector('german', coalesce((
+      SELECT string_agg(t.name, ' ')
+      FROM   document_tags dt
+      JOIN   tag t ON t.id = dt.tag_id
+      WHERE  dt.document_id = NEW.id
+    ), '')), 'D') ||
+    setweight(to_tsvector('german', coalesce(NEW.meta_location, '')), 'D');
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER trg_documents_fts
+  BEFORE INSERT OR UPDATE ON documents
+  FOR EACH ROW EXECUTE FUNCTION fn_documents_fts_update();
+
+-- 3. Rebuild trigger for join tables and transcription_blocks.
+--    These tables don't have a search_vector of their own; instead they
+--    touch the parent document row ("SET title = title") to re-fire the
+--    BEFORE UPDATE trigger above, which then recomputes the vector with
+--    the current state of all joined tables.
+CREATE OR REPLACE FUNCTION fn_rebuild_document_fts() RETURNS trigger AS $$
+DECLARE
+  v_doc_id UUID;
+BEGIN
+  v_doc_id := CASE WHEN TG_OP = 'DELETE' THEN OLD.document_id ELSE NEW.document_id END;
+  UPDATE documents SET title = title WHERE id = v_doc_id;
+  RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER trg_transcription_blocks_fts
+  AFTER INSERT OR UPDATE OR DELETE ON transcription_blocks
+  FOR EACH ROW EXECUTE FUNCTION fn_rebuild_document_fts();
+
+CREATE TRIGGER trg_document_receivers_fts
+  AFTER INSERT OR DELETE ON document_receivers
+  FOR EACH ROW EXECUTE FUNCTION fn_rebuild_document_fts();
+
+CREATE TRIGGER trg_document_tags_fts
+  AFTER INSERT OR DELETE ON document_tags
+  FOR EACH ROW EXECUTE FUNCTION fn_rebuild_document_fts();