feat(#65): implement full-text search with tsvector and GIN index

Add PostgreSQL full-text search infrastructure for knowledge entries:
- Add search_vector tsvector column to knowledge_entries table
- Create GIN index for fast full-text search performance
- Implement automatic trigger to maintain search_vector on insert/update
- Weight fields: title (A), summary (B), content (C)
- Update SearchService to use precomputed search_vector
- Add comprehensive integration tests for FTS functionality

Tests:
- 8/8 new integration tests passing
- 205/225 knowledge module tests passing
- All quality gates pass (typecheck, lint)

Refs #65

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/prisma/migrations/20260202142100_add_fulltext_search_to_knowledge_entries/migration.sql

@@ -0,0 +1,36 @@
+-- Add tsvector column for full-text search on knowledge_entries
+-- Weighted fields: title (A), summary (B), content (C)
+
+-- Step 1: Add the search_vector column
+ALTER TABLE "knowledge_entries"
+ADD COLUMN "search_vector" tsvector;
+
+-- Step 2: Create GIN index for fast full-text search
+CREATE INDEX "knowledge_entries_search_vector_idx"
+ON "knowledge_entries"
+USING gin("search_vector");
+
+-- Step 3: Create function to update search_vector
+CREATE OR REPLACE FUNCTION knowledge_entries_search_vector_update()
+RETURNS trigger AS $$
+BEGIN
+  NEW.search_vector :=
+    setweight(to_tsvector('english', COALESCE(NEW.title, '')), 'A') ||
+    setweight(to_tsvector('english', COALESCE(NEW.summary, '')), 'B') ||
+    setweight(to_tsvector('english', COALESCE(NEW.content, '')), 'C');
+  RETURN NEW;
+END
+$$ LANGUAGE plpgsql;
+
+-- Step 4: Create trigger to automatically update search_vector on insert/update
+CREATE TRIGGER knowledge_entries_search_vector_trigger
+BEFORE INSERT OR UPDATE ON "knowledge_entries"
+FOR EACH ROW
+EXECUTE FUNCTION knowledge_entries_search_vector_update();
+
+-- Step 5: Populate search_vector for existing entries
+UPDATE "knowledge_entries"
+SET search_vector =
+  setweight(to_tsvector('english', COALESCE(title, '')), 'A') ||
+  setweight(to_tsvector('english', COALESCE(summary, '')), 'B') ||
+  setweight(to_tsvector('english', COALESCE(content, '')), 'C');

apps/api/prisma/schema.prisma

@@ -798,6 +798,9 @@ model KnowledgeEntry {
   contentHtml String? @map("content_html") @db.Text
   summary     String?
 
+  // Full-text search vector (automatically maintained by trigger)
+  searchVector Unsupported("tsvector")? @map("search_vector")
+
   // Status
   status     EntryStatus @default(DRAFT)
   visibility Visibility  @default(PRIVATE)
@@ -820,6 +823,7 @@ model KnowledgeEntry {
   @@index([workspaceId, updatedAt])
   @@index([createdBy])
   @@index([updatedBy])
+  // Note: GIN index on searchVector created via migration (not supported in Prisma schema)
   @@map("knowledge_entries")
 }