docs: document cache_max_bytes, update TODO.md (closes #51)

Add cache_max_bytes to config.example.yml and the README key settings
list. TODO.md: move cache size management and eviction to Completed
Steps, promote P1 blocked networks configuration into Next Step, and
note in Status that the unbounded disk growth DoS vector is closed.

config.example.yml

@@ -28,6 +28,13 @@ allow_http: false
 # Maximum concurrent connections per upstream host (default: 20)
 upstream_connections_per_host: 20
 
+# Maximum disk cache size in bytes. Explicit values are used exactly as
+# given; 0 disables the disk cache entirely (every request fetches and
+# processes uncached). When omitted, the default is 75% of the free
+# space on the filesystem containing <state_dir>/cache/ at startup,
+# with a minimum of 500 MiB.
+# cache_max_bytes: 10737418240
+
 # Sentry error reporting (optional)
 sentry_dsn: ""