docs: add proxy source API documentation

docs/USAGE.md

@@ -18,6 +18,9 @@ s5p -l 0.0.0.0:9999 -C socks5://127.0.0.1:9050
 # From config file
 s5p -c config/s5p.yaml
 
+# With proxy source API (rotate exit proxy per-connection)
+s5p -C socks5://127.0.0.1:9050 -S http://10.200.1.250:8081/proxies
+
 # Debug mode
 s5p -v -C socks5://127.0.0.1:9050
 ```
@@ -42,7 +45,13 @@ log_level: info
 
 chain:
   - socks5://127.0.0.1:9050
-  - http://user:pass@proxy:8080
+
+proxy_source:
+  url: http://10.200.1.250:8081/proxies
+  proto: socks5       # optional: filter by protocol
+  country: US         # optional: filter by country
+  limit: 1000         # max proxies to fetch
+  refresh: 300        # cache refresh interval (seconds)
 ```
 
 ## Proxy URL Format
@@ -69,6 +78,29 @@ make down    # stop and remove container
 Source (`./src`) and config (`./config/s5p.yaml`) are mounted read-only
 into the container. Edit locally, restart to pick up changes.
 
+## Proxy Source
+
+Appends a random proxy from an HTTP API after the static chain on each
+connection. Proxies are cached and refreshed at a configurable interval.
+
+```yaml
+proxy_source:
+  url: http://10.200.1.250:8081/proxies
+  proto: socks5       # optional: only fetch this protocol
+  country: US         # optional: only fetch this country
+  limit: 1000         # max proxies to fetch from API
+  refresh: 300        # re-fetch every 300 seconds
+```
+
+CLI shorthand (uses defaults for limit/refresh):
+
+```bash
+s5p -C socks5://127.0.0.1:9050 -S http://10.200.1.250:8081/proxies
+```
+
+The API must return JSON: `{"proxies": [{"proto": "socks5", "proxy": "host:port"}, ...]}`.
+Entries with `null` proto are skipped.
+
 ## Profiling
 
 ```bash