Trust proxy headers by hop count, and log every credential event

Operator, before exposing the instance: *"I'd expect that we should have a proxy
hops setting for how many proxy hops we should trust a shared real-ip at… and is
there any session logging."* Neither existed, and the first one was a real hole.

**The address was forgeable.** `client_address()` read the LEFTMOST
`X-Forwarded-For` entry — nominally "the original client", and precisely the one a
caller controls, because anything they send arrives before what proxies append. So
`curl -H "X-Forwarded-For: 1.2.3.4"`, rotated per request, minted a fresh
rate-limit bucket every time.

Concretely: stuffing ONE account stayed limited (the account key is unforgeable
and that is why it exists), but spraying MANY accounts from one source was not —
each account got its own budget, and the per-address cap meant to bound the total
was defeated by a header. On a LAN that is nothing. It is not nothing on a public
host.

Now it counts in from the RIGHT by `THOUGHTSYNC_TRUSTED_PROXY_HOPS`, default 1.
Each hop appends what it saw, so the rightmost entries are the ones our own
infrastructure wrote and a forged prefix lands to the left of them where it can
never be selected — proven for the honest, forged, padded, CDN and
shorter-than-configured cases. 0 ignores the header entirely; 2 is Cloudflare in
front of a proxy. Too high is the dangerous direction, so a header shorter than
configured falls back to the socket address rather than reaching further left.

`X-Forwarded-Proto` had the same bug and now shares the same rule. Both live in a
new `proxy.py` rather than being written twice — two places holding one decision
is how issue 2183 happened, and this is the same decision.

Env rather than the Settings UI, against rule 25's usual pull: it is deployment
topology rather than preference, and the limiter consults it BEFORE opening a
database connection, which is the entire point of checking a throttle before doing
expensive work. Easy to move if that reads wrong.

**And there was no logging at all** — `auth.py` had no logger, and the only record
of anything was `device_tokens.last_used_at`. Sign-ins, failures, throttle trips,
new accounts and device-token issuance now all log, with the attempted email and
the trusted address. Deliberately including the email: it is the operator's own
server, and "somebody failed a login" without saying against which account is not
actionable.

`basicConfig` at INFO in `create_app`, because hypercorn configures its own loggers
and leaves the root at WARNING — without it every line above would have gone
nowhere, which is a worse failure than not writing them.

This is the app log, not an audit table. Not queryable, not retained past log
rotation. The table is task 2939; this is what makes the next few days observable.

.env.example

@@ -39,6 +39,24 @@ POSTGRES_PASSWORD=
 # 127.0.0.1 so only the proxy can talk to it.
 #THOUGHTSYNC_BIND=0.0.0.0
 
+# How many proxies sit in front of this app. This is a SECURITY setting, not a
+# preference: it decides which entry of X-Forwarded-For is believed, and therefore
+# whether a caller can forge their own address and slip the per-address rate limit.
+#
+#   0  nothing in front — the app is directly exposed
+#   1  one reverse proxy terminating TLS (the default, and the usual case)
+#   2  a CDN in front of that proxy, e.g. Cloudflare
+#
+# Set it to the number you actually run. Too HIGH is the dangerous direction — it
+# starts trusting entries no proxy of yours wrote. Too low just means callers share
+# a rate-limit bucket.
+#THOUGHTSYNC_TRUSTED_PROXY_HOPS=1
+
+# How much the app says. Credential events (sign-ins, failures, throttles, new
+# accounts, device tokens issued) are logged at INFO and read with
+# `docker compose logs app`.
+#THOUGHTSYNC_LOG_LEVEL=INFO
+
 # Database identity. Changing these AFTER the first start does not rename anything
 # that already exists — the volume keeps whatever the first run created.
 #POSTGRES_USER=thoughtsync