docs: add url shortener documentation

SECURITY.md

@@ -92,6 +92,25 @@ FLASKPASTE_MIN_ENTROPY=6.0      # Bits per byte (encrypted ~7.5-8.0)
 FLASKPASTE_MIN_ENTROPY_SIZE=256 # Only check content >= this size
 ```
 
+### URL Shortener Security
+
+**Open Redirect Prevention**
+
+Short URL creation validates target URLs:
+
+- Only `http` and `https` schemes allowed (rejects `javascript:`, `data:`, `ftp:`, `file:`)
+- Network location (hostname) required — rejects scheme-only URLs
+- Maximum URL length: 2048 bytes
+- Short IDs: 8-char base62 (`[a-zA-Z0-9]`) with `secrets.choice()` for unpredictability
+- Redirect responses include `Cache-Control: no-store, no-cache` to prevent caching
+
+**Access Controls**
+
+- Creation: rate-limited + proof-of-work (same as paste creation)
+- Redirect: lookup rate limiting prevents enumeration
+- Deletion: owner authentication required
+- Listing: authentication required, shows only own URLs
+
 ### Security Headers
 
 All responses include:
@@ -120,6 +139,12 @@ All requests receive `X-Request-ID` header for log correlation and debugging. Pa
 - Configurable length (default 12 characters)
 - Validated on all endpoints
 
+### Short URL IDs
+
+- Base62 only (`[a-zA-Z0-9]+`)
+- 8 characters (configurable via `FLASKPASTE_SHORT_ID_LENGTH`)
+- Validated on all `/s/` endpoints
+
 ### MIME Types
 
 - Magic byte detection for binary formats
@@ -245,6 +270,7 @@ Security fixes are released as soon as possible. Subscribe to repository release
 - Authentication bypass
 - Information disclosure
 - Denial of service (application-level)
+- Open redirect via URL shortener
 
 ### Out of Scope
 
@@ -258,6 +284,7 @@ Security fixes are released as soon as possible. Subscribe to repository release
 
 | Version | Security Changes |
 |---------|------------------|
+| 1.6.0   | URL shortener with open redirect prevention, scheme allowlist, target URL validation |
 | 1.5.0   | Pentest remediation (15 items): timing attack prevention, serial collision detection, lookup rate limiting, content hash locking, anti-flood memory limits, CLI path validation, SSL hostname verification, config permission checks |
 | 1.4.0   | Anti-flood dynamic PoW, IP-based rate limiting, audit logging |
 | 1.2.0   | Password protection with PBKDF2, code modernization |