Stop throttling signed media and ease the browser's e621 queue

Signed media URLs are fetched by <img>/<video> tags without an
Authorization header, so they were charged to the anonymous 120/min
bucket: past that, galleries and the fish-greeting download got 429 JSON
instead of image bytes. The raw/thumbnail/staged-file/similarity-file
actions are now exempt, and THROTTLE_ENABLED=false removes the general
anon+user limits for private/tailnet deployments (login/register/proxy
guards stay).

The SPA's e621 client also stops self-throttling so hard: 1s gap between
browsing calls (2.5s for the stricter IQDB endpoint) and a 15s cooldown
instead of 60s when e621 answers 429.

deploy/.env.example

@@ -64,6 +64,10 @@ DB_ROOT_PASSWORD=j621root
 # THROTTLE_LOGIN=5/min
 # THROTTLE_REGISTER=20/hour
 # THROTTLE_E621_PROXY=60/hour
+# Tailnet-only / private deployments can drop the general limits entirely.
+# Signed media URLs (<img>/<video>) and the login/register/proxy guards are
+# exempt from this switch either way.
+# THROTTLE_ENABLED=false
 
 # e621 media hosts the backend may fetch from (downloads, proxies)
 # E621_MEDIA_HOSTS=static1.e621.net,static2.e621.net,static3.e621.net