fix(api): KB 架構審查修復 I3-I5

- I3: Service 層加 IKnowledgeRepository Protocol 型別標注
- I4: search 方法加入 tags JSONB 搜尋 (cast→String→ilike)
- I5: get_categories 獨立方法，不再繞道 list_entries(limit=0)

首席架構師審查 87/100 → 全部 Important issues 已修復

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/api/src/repositories/knowledge_repository.py

@@ -12,7 +12,7 @@ Knowledge Base Phase 1: CRUD + 搜尋
 """
 
 import structlog
-from sqlalchemy import func, or_, select, update
+from sqlalchemy import String, func, or_, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from src.db.models import KnowledgeEntryRecord
@@ -154,7 +154,7 @@ class KnowledgeDBRepository:
         return [(row.category, row.cnt) for row in result.all()]
 
     async def search(self, query: str, limit: int = 20) -> list[KnowledgeEntry]:
-        """關鍵字搜尋 (title + content)"""
+        """關鍵字搜尋 (title + content + tags)"""
         like_q = f"%{query}%"
         result = await self.db.execute(
             select(KnowledgeEntryRecord)
@@ -163,6 +163,7 @@ class KnowledgeDBRepository:
                 or_(
                     KnowledgeEntryRecord.title.ilike(like_q),
                     KnowledgeEntryRecord.content.ilike(like_q),
+                    KnowledgeEntryRecord.tags.cast(String).ilike(like_q),
                 ),
             )
             .order_by(KnowledgeEntryRecord.view_count.desc())