feat: add darkMode and hideSelectors screenshot parameters

- darkMode: emulates prefers-color-scheme: dark before navigation
- hideSelectors: injects CSS to hide elements before capture
  - POST: accepts string or string array
  - GET: accepts comma-separated string
  - Validation: max 10 selectors, each max 200 chars
- OpenAPI docs updated for both GET and POST endpoints
- 13 new tests added (service + route)

src/services/__tests__/screenshot.test.ts

@@ -24,6 +24,8 @@ function createMockPage() {
     goto: vi.fn().mockResolvedValue(undefined),
     waitForSelector: vi.fn().mockResolvedValue(undefined),
     screenshot: vi.fn().mockResolvedValue(Buffer.from('fake-screenshot')),
+    emulateMediaFeatures: vi.fn().mockResolvedValue(undefined),
+    addStyleTag: vi.fn().mockResolvedValue(undefined),
   }
 }
 
@@ -177,6 +179,67 @@ describe('Screenshot Service', () => {
     })
   })
 
+  describe('darkMode', () => {
+    it('emulates prefers-color-scheme: dark when darkMode is true', async () => {
+      await takeScreenshot({ url: 'https://example.com', darkMode: true })
+      expect(mockPage.emulateMediaFeatures).toHaveBeenCalledWith([
+        { name: 'prefers-color-scheme', value: 'dark' }
+      ])
+    })
+
+    it('calls emulateMediaFeatures before goto', async () => {
+      const callOrder: string[] = []
+      mockPage.emulateMediaFeatures.mockImplementation(async () => { callOrder.push('emulate') })
+      mockPage.goto.mockImplementation(async () => { callOrder.push('goto') })
+      await takeScreenshot({ url: 'https://example.com', darkMode: true })
+      expect(callOrder).toEqual(['emulate', 'goto'])
+    })
+
+    it('does not emulate media features when darkMode is false', async () => {
+      await takeScreenshot({ url: 'https://example.com', darkMode: false })
+      expect(mockPage.emulateMediaFeatures).not.toHaveBeenCalled()
+    })
+
+    it('does not emulate media features when darkMode is not set', async () => {
+      await takeScreenshot({ url: 'https://example.com' })
+      expect(mockPage.emulateMediaFeatures).not.toHaveBeenCalled()
+    })
+  })
+
+  describe('hideSelectors', () => {
+    it('injects style tag to hide selectors after page load', async () => {
+      await takeScreenshot({ url: 'https://example.com', hideSelectors: ['.ad', '#banner'] })
+      expect(mockPage.addStyleTag).toHaveBeenCalledWith({
+        content: '.ad { display: none !important }\n#banner { display: none !important }'
+      })
+    })
+
+    it('calls addStyleTag after goto', async () => {
+      const callOrder: string[] = []
+      mockPage.goto.mockImplementation(async () => { callOrder.push('goto') })
+      mockPage.addStyleTag.mockImplementation(async () => { callOrder.push('addStyleTag') })
+      await takeScreenshot({ url: 'https://example.com', hideSelectors: ['.ad'] })
+      expect(callOrder.indexOf('goto')).toBeLessThan(callOrder.indexOf('addStyleTag'))
+    })
+
+    it('does not inject style tag when hideSelectors is empty', async () => {
+      await takeScreenshot({ url: 'https://example.com', hideSelectors: [] })
+      expect(mockPage.addStyleTag).not.toHaveBeenCalled()
+    })
+
+    it('does not inject style tag when hideSelectors is not set', async () => {
+      await takeScreenshot({ url: 'https://example.com' })
+      expect(mockPage.addStyleTag).not.toHaveBeenCalled()
+    })
+
+    it('handles single selector', async () => {
+      await takeScreenshot({ url: 'https://example.com', hideSelectors: ['.cookie-banner'] })
+      expect(mockPage.addStyleTag).toHaveBeenCalledWith({
+        content: '.cookie-banner { display: none !important }'
+      })
+    })
+  })
+
   describe('Page lifecycle', () => {
     it('always releases page after successful screenshot', async () => {
       await takeScreenshot({ url: 'https://example.com' })