docs: modernize and clean deprecated content

- replace deprecated FLASK_ENV with FLASK_DEBUG
- remove duplicate FLASKPASTE_MAX_EXPIRY entry
- update API version to 1.5.0
- add missing /pastes and /pki endpoints to table
- remove deprecated X-XSS-Protection header
- add PKI config variables
- update features list with current capabilities
- update auth benefits and security sections

documentation/api.md

@@ -22,7 +22,9 @@ The fingerprint must be exactly 40 lowercase hexadecimal characters (SHA1).
 
 **Benefits of authentication:**
 - Larger upload limit (50 MiB vs 3 MiB)
-- Ability to delete owned pastes
+- Longer default expiry (7-30 days vs 1 day)
+- Ability to list and delete owned pastes
+- Register via PKI for trusted status and admin eligibility
 
 ---
 
@@ -120,7 +122,7 @@ Host: localhost:5000
 ```json
 {
   "name": "FlaskPaste",
-  "version": "1.2.0",
+  "version": "1.5.0",
   "endpoints": {
     "GET /": "API information",
     "GET /health": "Health check",
@@ -795,7 +797,6 @@ All responses include the following security headers:
 | `Cache-Control` | `no-store, no-cache, must-revalidate, private` |
 | `Pragma` | `no-cache` |
 
-
 ---
 
 ## Request Tracing