Add diagram/doc for zip streaming

docs/zip-download-flow.md

@@ -0,0 +1,56 @@
+# Zip Download Flow
+
+This diagram illustrates the flow for downloading images as a zip file.
+
+```mermaid
+sequenceDiagram
+    autonumber
+    Client->>+API: Generate a signed URL for zip download
+    API-->>-Client: Signed Zip Server URL
+    Client->>+Zip Server: Request Zip with signed URL
+    Zip Server->>+API: Ask for Zip File Descriptor
+    API-->>-Zip Server: Zip file descriptor
+    par
+        Zip Server->>+S3: Get image files
+    and
+        Zip Server->>+API: Get metadata/attribution/license files
+    and
+        Zip Server-->>-Client: Stream zip file
+    end
+```
+
+## Security
+
+### Step 1: Client → API (Generate signed URL)
+- User's identity and search parameters are captured
+
+### Step 2: API → Client (Return signed URL)
+- URL contains a cryptographically signed token (Django `TimestampSigner`)
+- Token expires after 1 day
+- Token encodes: user ID, search query, collection filters
+
+### Step 3: Client → Zip Server (Request zip)
+- Client opaquely passes the signed token to the external zip server
+- Token serves as proof of authorization
+
+### Step 4: Zip Server → API (Request file descriptor)
+- Zip server is hardcoded to retrieve the descriptor from the one true API server with the given zsid
+- Token validation via `ZipDownloadTokenAuth`
+- Verifies signature and checks 24-hour expiration
+
+### Step 5: API → Zip Server (Return file descriptor)
+- API returns file listing only for images the user is authorized to access
+- Descriptor contains bare unsigned S3 URLs for images and API URLs for metadata files
+- Query is executed with user's permissions applied
+
+### Step 6: Zip Server → S3 (Fetch images)
+- **Public images** (sponsored bucket):
+  - Bucket has public read policy
+- **Private images** (default bucket):
+  - Bucket and all objects are private
+  - Zip server has the an instance profile granting explicit read access
+
+### Step 7: Zip Server → API (Fetch metadata/attribution/license files)
+- Same token authentication as Step 4
+- Files are generated on-the-fly
+- Each request validates the token

isic/settings/base.py

@@ -159,6 +159,10 @@
 
 # Disallowing CORS credentials is all the security necessary.
 # The API is safe to call by anyone, as it has no side effects.
+# Note: CORS_ALLOW_ALL_ORIGINS (which sets Access-Control-Allow-Origin: *)
+# cannot be used with credentials per the CORS spec. Browsers will block any
+# request that attempts credentials: 'include' when the response has a wildcard
+# origin, preventing CSRF attacks even on @csrf_exempt endpoints.
 CORS_ALLOW_ALL_ORIGINS = True
 
 PASSWORD_HASHERS += [