Correct trusted_proxies advice and state signature padding (closes #150)

The README told operators to set trusted_proxies to the proxy's own
address. A proxy on the Docker host that connects over 127.0.0.1 reaches
pixa from the Docker network's gateway, so that advice made pixa count
every user as one client for the login limit. The login-limit paragraph,
the trusted_proxies entry and config.example.yml now say to use the
address pixa sees for requests through the proxy, that a proxy connecting
through another host address is seen with that address, and how to read
it from the request log.

The signature section now says sig is base64url with the = padding
kept, since pixa compares it exactly, and shows the example's sig for a
stated key, computed with pixa's signer.

Model: opus-5-5

config.example.yml

@@ -50,7 +50,13 @@ allowlist_hosts:
 # 172.16.0.0/12, 192.168.0.0/16), since pixa is deployed behind a proxy on
 # a private network. An explicitly empty list ([]) trusts no one; an
 # explicit list replaces the default. An invalid CIDR aborts startup.
-# Uncomment to override the defaults with your proxy's address range.
+# Uncomment to override the defaults with the address pixa sees for
+# requests that come through your proxy. That is not always the proxy's own
+# address: a proxy on the Docker host that connects over 127.0.0.1 is seen
+# as the gateway of the container's Docker network (172.17.0.1 on the
+# default bridge), and one that connects through another host address is
+# seen with that address. To be sure, look it up in the request log as the
+# trusted_proxies entry in README.md describes.
 # trusted_proxies:
 #   - 10.0.0.0/8
 #   - 2001:db8::/32