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.

src/thoughtsync/app.py

@@ -1,13 +1,14 @@
 from __future__ import annotations
 
 import asyncio
+import logging
 import mimetypes
 import os
 import secrets
 from contextlib import suppress
 from datetime import timedelta
 
-from quart import Quart, has_request_context, jsonify, request, send_from_directory
+from quart import Quart, jsonify, send_from_directory
 from quart.sessions import SecureCookieSessionInterface
 
 from . import __version__
@@ -17,12 +18,25 @@ from .config import Config
 from .db import session_scope
 from .labels import bp as labels_bp
 from .notes import bp as notes_bp
+from .proxy import is_https
 from .retention import run_sweeper
 from .saved_filters import bp as saved_filters_bp
 from .settings import get_public_config, get_setting, load_or_create_secret_key
 from .settings_api import bp as settings_bp
 from .sync import bp as sync_bp, protocol_advertisement
 
+# Without this, `logger.info` from this package goes nowhere: hypercorn configures its
+# own access/error loggers and leaves the root logger at WARNING, so the credential
+# events in auth.py would be invisible in `docker compose logs` — which is exactly
+# where they are meant to be read until an audit table exists (task 2939).
+#
+# `force=False` (the default) so a host that has already configured logging keeps its
+# own setup; LOG_LEVEL lets an operator turn it up without a code change.
+logging.basicConfig(
+    level=os.environ.get("THOUGHTSYNC_LOG_LEVEL", "INFO").upper(),
+    format="%(asctime)s %(levelname)s %(name)s %(message)s",
+)
+
 STATIC_DIR = os.path.join(os.path.dirname(__file__), "static")
 
 # `.webmanifest` isn't in every base image's mime map; register it so the PWA
@@ -30,19 +44,6 @@ STATIC_DIR = os.path.join(os.path.dirname(__file__), "static")
 mimetypes.add_type("application/manifest+json", ".webmanifest")
 
 
-def _is_https() -> bool:
-    """Whether this request reached us over TLS — directly, or through a proxy that
-    terminated it and said so in X-Forwarded-Proto.
-
-    Shared by the session cookie's Secure flag and by HSTS, because they are the same
-    question and answering it twice is how the two drift apart.
-    """
-    if not has_request_context():
-        return False
-    forwarded = request.headers.get("X-Forwarded-Proto", "").split(",")[0].strip().lower()
-    return forwarded == "https" or request.is_secure
-
-
 class _AutoSecureSessionInterface(SecureCookieSessionInterface):
     """Mark the session cookie `Secure` whenever the request arrived over HTTPS —
     directly, or via a TLS-terminating reverse proxy that sets X-Forwarded-Proto.
@@ -54,7 +55,7 @@ class _AutoSecureSessionInterface(SecureCookieSessionInterface):
     """
 
     def get_cookie_secure(self, app: Quart) -> bool:
-        return _is_https()
+        return is_https()
 
 
 def create_app() -> Quart:
@@ -168,7 +169,7 @@ def create_app() -> Quart:
         # commit domains this app does not own. A browser still remembers the policy
         # for up to a year after the header stops being sent, which is the point of
         # it — worth knowing before putting a hostname behind TLS temporarily.
-        if _is_https():
+        if is_https():
             response.headers.setdefault("Strict-Transport-Security", "max-age=31536000")
         return response