feat(04-01): add tags schema with case-insensitive unique index

- Add tags table with nanoid PK and case-insensitive unique index on name
- Add entry_tags junction table with composite PK and cascade deletes
- Export lower() helper function for case-insensitive queries
- Export Tag, NewTag, EntryTag types for TypeScript inference

src/lib/server/db/schema.ts

@@ -1,4 +1,11 @@
-import { sqliteTable, text, integer } from 'drizzle-orm/sqlite-core';
+import { sqliteTable, text, integer, uniqueIndex, primaryKey } from 'drizzle-orm/sqlite-core';
+import { sql, type SQL } from 'drizzle-orm';
+import type { AnySQLiteColumn } from 'drizzle-orm/sqlite-core';
+
+// Helper function for case-insensitive indexing
+export function lower(column: AnySQLiteColumn): SQL {
+	return sql`lower(${column})`;
+}
 
 // Entry types: 'task' (actionable) or 'thought' (reference)
 export const entries = sqliteTable('entries', {
@@ -36,3 +43,35 @@ export const images = sqliteTable('images', {
 
 export type Image = typeof images.$inferSelect;
 export type NewImage = typeof images.$inferInsert;
+
+// Tags table: reusable tags for organizing entries
+export const tags = sqliteTable(
+	'tags',
+	{
+		id: text('id').primaryKey(), // nanoid
+		name: text('name').notNull(),
+		createdAt: text('created_at')
+			.notNull()
+			.$defaultFn(() => new Date().toISOString())
+	},
+	(table) => [uniqueIndex('tagNameUniqueIndex').on(lower(table.name))]
+);
+
+export type Tag = typeof tags.$inferSelect;
+export type NewTag = typeof tags.$inferInsert;
+
+// Junction table for many-to-many entry-tag relationship
+export const entryTags = sqliteTable(
+	'entry_tags',
+	{
+		entryId: text('entry_id')
+			.notNull()
+			.references(() => entries.id, { onDelete: 'cascade' }),
+		tagId: text('tag_id')
+			.notNull()
+			.references(() => tags.id, { onDelete: 'cascade' })
+	},
+	(t) => [primaryKey({ columns: [t.entryId, t.tagId] })]
+);
+
+export type EntryTag = typeof entryTags.$inferSelect;