feat(04-01): create tagRepository with tag operations

- Add TagRepository interface with findOrCreate, getAll, getById, getByEntryId, updateEntryTags
- Implement SQLiteTagRepository with case-insensitive tag lookup
- updateEntryTags atomically replaces all tags for an entry
- Export tagRepository singleton

src/lib/server/db/repository.ts

@@ -1,6 +1,17 @@
-import { eq, desc, asc, ne } from 'drizzle-orm';
+import { eq, desc, asc, ne, sql } from 'drizzle-orm';
 import { db } from './index';
-import { entries, images, type Entry, type NewEntry, type Image, type NewImage } from './schema';
+import {
+	entries,
+	images,
+	tags,
+	entryTags,
+	lower,
+	type Entry,
+	type NewEntry,
+	type Image,
+	type NewImage,
+	type Tag
+} from './schema';
 import { nanoid } from 'nanoid';
 
 export interface EntryRepository {
@@ -139,3 +150,71 @@ class SQLiteImageRepository implements ImageRepository {
 }
 
 export const imageRepository: ImageRepository = new SQLiteImageRepository();
+
+// Tag Repository
+export interface TagRepository {
+	findOrCreate(name: string): Tag;
+	getAll(): Tag[];
+	getById(id: string): Tag | undefined;
+	getByEntryId(entryId: string): Tag[];
+	updateEntryTags(entryId: string, tagNames: string[]): void;
+}
+
+class SQLiteTagRepository implements TagRepository {
+	findOrCreate(name: string): Tag {
+		const normalizedName = name.trim();
+
+		// Try to find existing tag (case-insensitive)
+		const existing = db
+			.select()
+			.from(tags)
+			.where(sql`lower(${tags.name}) = lower(${normalizedName})`)
+			.get();
+
+		if (existing) return existing;
+
+		// Create new tag
+		const newTag: Tag = {
+			id: nanoid(),
+			name: normalizedName,
+			createdAt: new Date().toISOString()
+		};
+		db.insert(tags).values(newTag).run();
+		return newTag;
+	}
+
+	getAll(): Tag[] {
+		return db.select().from(tags).orderBy(asc(tags.name)).all();
+	}
+
+	getById(id: string): Tag | undefined {
+		return db.select().from(tags).where(eq(tags.id, id)).get();
+	}
+
+	getByEntryId(entryId: string): Tag[] {
+		return db
+			.select({
+				id: tags.id,
+				name: tags.name,
+				createdAt: tags.createdAt
+			})
+			.from(entryTags)
+			.innerJoin(tags, eq(entryTags.tagId, tags.id))
+			.where(eq(entryTags.entryId, entryId))
+			.orderBy(asc(tags.name))
+			.all();
+	}
+
+	updateEntryTags(entryId: string, tagNames: string[]): void {
+		// Delete existing associations
+		db.delete(entryTags).where(eq(entryTags.entryId, entryId)).run();
+
+		// Find or create each tag and create associations
+		for (const name of tagNames) {
+			const tag = this.findOrCreate(name);
+			db.insert(entryTags).values({ entryId, tagId: tag.id }).run();
+		}
+	}
+}
+
+export const tagRepository: TagRepository = new SQLiteTagRepository();