Add snapshot management to API and enhance user-agent validation for v2 routes

README.md

@@ -15,6 +15,7 @@ Generate Twitter-style quote images programmatically. Perfect for creating socia
 - Base64 image support
 - Docker-ready
 - v2 API with stateful sessions for incremental updates
+- Persistent snapshot links (48-hour TTL, immutable, shareable)
 
 ## API Endpoints
 
@@ -323,6 +324,47 @@ curl -X DELETE https://quotes.imbenji.net/v2/quote/a1b2c3d4-e5f6-7890-abcd-ef123
 
 Returns `204 No Content` on success.
 
+### POST /v2/quote/:id/snapshot-link
+
+Create a persistent snapshot link. This captures the current state of your session and generates a shareable URL that persists for 48 hours (refreshing on each access).
+
+Unlike the regular `/image` endpoint, snapshots are immutable - they always show the image as it was when the snapshot was created, even if you update the session afterwards.
+
+**Request:**
+```bash
+curl -X POST https://quotes.imbenji.net/v2/quote/a1b2c3d4-e5f6-7890-abcd-ef1234567890/snapshot-link
+```
+
+**Response (201):**
+```json
+{
+  "token": "xY9pQmN3kL8vFw2jRtZ7",
+  "url": "https://quotes.imbenji.net/v2/snapshot/xY9pQmN3kL8vFw2jRtZ7",
+  "sessionId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "createdAt": 1735600000,
+  "expiresAt": 1735772800
+}
+```
+
+### GET /v2/snapshot/:token
+
+Retrieve a snapshot image using its token.
+
+```bash
+curl https://quotes.imbenji.net/v2/snapshot/xY9pQmN3kL8vFw2jRtZ7 --output snapshot.png
+```
+
+**Response headers:**
+- `Content-Type: image/png`
+- `X-Snapshot-Token: <token>`
+- `X-Cache: HIT` or `MISS`
+
+**Snapshot behavior:**
+- **Immutable:** Shows the session state when the snapshot was created
+- **TTL:** 48 hours from last access (resets on each view)
+- **Cascade delete:** Deleted automatically when parent session is deleted
+- **Shareable:** Token can be shared with anyone
+
 ### v2 Example Workflow
 
 ```javascript
@@ -360,6 +402,14 @@ await fetch(`https://quotes.imbenji.net/v2/quote/${sessionId}`, {
 // 4. Generate the final image
 const imgRes = await fetch(`https://quotes.imbenji.net/v2/quote/${sessionId}/image`);
 const blob = await imgRes.blob();
+
+// 5. (Optional) Create a shareable snapshot link
+const snapshotRes = await fetch(`https://quotes.imbenji.net/v2/quote/${sessionId}/snapshot-link`, {
+  method: 'POST'
+});
+const snapshot = await snapshotRes.json();
+console.log('Shareable URL:', snapshot.url);
+// Anyone can access: https://quotes.imbenji.net/v2/snapshot/<token>
 ```
 
 ### v2 Parameters
@@ -468,8 +518,9 @@ Response:
 - **Max tweet width:** 450px (auto-scaled to fit)
 - **Cache TTL:** 24 hours from last access
 - **Session TTL:** 24 hours from last update (v2 API)
+- **Snapshot TTL:** 48 hours from last access (v2 API)
 - **Cleanup interval:** Every hour
-- **Database:** SQLite (for v2 sessions)
+- **Database:** SQLite (for v2 sessions and snapshots)
 
 ## Limitations