feat: filter the storefront by favorited items (#35)

Adds favorites as another dimension of the existing storefront filter rather than a separate view, so it lives in the URL, shows up as a removable chip, and combines with category, tags, and price by AND like everything else. A customer can ask for "my favorites under $500 in Furniture" instead of only "my favorites".

Sold favorites are included. The storefront shows sold items everywhere else, and a favorite that has just sold is often exactly what the customer came back to look at after being emailed about it in #34. Hiding them would make items disappear from a list the customer curated themselves. Anyone wanting only what they can still buy can combine the toggle with the status filter.

Which customer "my favorites" means comes from the session, never from the query string, so a hand-edited URL cannot name someone else's favorites. A signed-out visitor sees the toggle and gets the same inline register/login prompt the heart button and Add to Cart already use; signing in resolves the gate and the filter applies on its own. A bookmarked favorites link whose session has expired says so rather than rendering an empty grid, which would tell the visitor they have no favorites instead of that we do not know who they are. The API answers 401 for the same reason, and the admin inventory refuses the filter outright rather than ignoring it.

The shared SQL builder now requires callers to say whose favorites they mean, even when that is nobody, and throws instead of dropping the clause — a future caller that forgets the guard fails loudly rather than quietly returning the whole catalogue.

Verified with 59 unit tests, 134 backend integration tests, and 70 end-to-end tests, all passing, with type checking clean on both sides.

backend/tests/unit/itemFilters.test.ts

@@ -7,7 +7,8 @@ describe('parseItemFilters', () => {
       tagIds: [],
       minPriceCents: null,
       maxPriceCents: null,
-      status: null
+      status: null,
+      favoritesOnly: false
     });
   });
 
@@ -85,6 +86,26 @@ describe('parseItemFilters', () => {
     expect(parseItemFilters({ status: '' }).status).toBeNull();
   });
 
+  it('parses the favorites flag in both spellings', () => {
+    expect(parseItemFilters({ favorites: '1' }).favoritesOnly).toBe(true);
+    expect(parseItemFilters({ favorites: 'true' }).favoritesOnly).toBe(true);
+  });
+
+  it('treats an explicit off value as off', () => {
+    expect(parseItemFilters({ favorites: '0' }).favoritesOnly).toBe(false);
+    expect(parseItemFilters({ favorites: 'false' }).favoritesOnly).toBe(false);
+    expect(parseItemFilters({ favorites: '' }).favoritesOnly).toBe(false);
+  });
+
+  it('defaults favorites to off', () => {
+    expect(parseItemFilters({}).favoritesOnly).toBe(false);
+  });
+
+  it('rejects a favorites value that is neither on nor off', () => {
+    expect(() => parseItemFilters({ favorites: 'yes' })).toThrow(FilterError);
+    expect(() => parseItemFilters({ favorites: 'mine' })).toThrow(FilterError);
+  });
+
   it('rejects a status outside the known set', () => {
     expect(() => parseItemFilters({ status: 'pending' })).toThrow(FilterError);
   });
@@ -96,19 +117,19 @@ describe('parseItemFilters', () => {
 
 describe('buildItemFilterSql', () => {
   it('produces no clauses and no params when nothing is filtered', () => {
-    const built = buildItemFilterSql(parseItemFilters({}), 1);
+    const built = buildItemFilterSql(parseItemFilters({}), 1, null);
     expect(built.clauses).toEqual([]);
     expect(built.params).toEqual([]);
   });
 
   it('matches a category and all of its descendants', () => {
-    const built = buildItemFilterSql(parseItemFilters({ category: '4' }), 1);
+    const built = buildItemFilterSql(parseItemFilters({ category: '4' }), 1, null);
     expect(built.clauses.join(' ')).toContain('RECURSIVE');
     expect(built.params).toEqual([4]);
   });
 
   it('requires every listed tag rather than any of them', () => {
-    const built = buildItemFilterSql(parseItemFilters({ tags: '1,2' }), 1);
+    const built = buildItemFilterSql(parseItemFilters({ tags: '1,2' }), 1, null);
     // The count of matched tag rows must equal the number of tags requested —
     // an ANY/IN match alone would return items carrying just one of them.
     expect(built.clauses.join(' ')).toContain('COUNT(*)');
@@ -116,20 +137,41 @@ describe('buildItemFilterSql', () => {
   });
 
   it('numbers placeholders from the given starting index', () => {
-    const built = buildItemFilterSql(parseItemFilters({ category: '4' }), 3);
+    const built = buildItemFilterSql(parseItemFilters({ category: '4' }), 3, null);
     expect(built.clauses.join(' ')).toContain('$3');
   });
 
   it('filters on status', () => {
-    const built = buildItemFilterSql(parseItemFilters({ status: 'reserved' }), 1);
+    const built = buildItemFilterSql(parseItemFilters({ status: 'reserved' }), 1, null);
     expect(built.clauses.join(' ')).toContain('i.status');
     expect(built.params).toEqual(['reserved']);
   });
 
+  it('restricts to the favorites of the given customer', () => {
+    const built = buildItemFilterSql(parseItemFilters({ favorites: '1' }), 1, 42);
+    expect(built.clauses.join(' ')).toContain('EXISTS');
+    expect(built.clauses.join(' ')).toContain('favorites f');
+    expect(built.params).toEqual([42]);
+  });
+
+  it('does not restrict to favorites when the flag is off, even given a customer', () => {
+    const built = buildItemFilterSql(parseItemFilters({}), 1, 42);
+    expect(built.clauses).toEqual([]);
+    expect(built.params).toEqual([]);
+  });
+
+  // Both routes reject this before reaching the builder, so it can only happen
+  // through a new caller that forgot to. Failing loudly beats dropping the
+  // clause and returning the whole catalogue as if it were someone's favorites.
+  it('throws rather than ignore a favorites filter with no customer', () => {
+    expect(() => buildItemFilterSql(parseItemFilters({ favorites: '1' }), 1, null)).toThrow();
+  });
+
   it('continues numbering across multiple filters', () => {
     const built = buildItemFilterSql(
       parseItemFilters({ category: '4', min_price: '100', max_price: '900' }),
-      1
+      1,
+      null
     );
     expect(built.params).toEqual([4, 100, 900]);
     const sql = built.clauses.join(' ');