feat(settings): the instance reports which build it is (318 step 6)

A dim line at the foot of Settings: `FabledCurator 2026.08.28.1249 · dev`.

This is no longer a convenience. Milestone 318 stopped publishing version
image tags, so an instance's own report is the ONLY answer to "which build is
this?" — there is no registry name left to check it against. Note #3127 §5
says it directly: a wrong answer here has no second source to contradict it.

Three states, kept distinct because collapsing any two of them lies:

  not asked yet         render nothing
  asked, no version     render "unknown"
  asked, has a version  render it

A blank footer reads as "no version", which is a different claim from "I
cannot say". And a failed health call deliberately does NOT mark the build
loaded — a network blip says nothing about the image, and presenting it as
"unknown" would look like a defective build.

Carried on /api/health rather than a new route: it answers at the same cost
(two module constants, no I/O) and TopNav already fetches it app-wide, so a
separate endpoint would mean a second request for two strings.

Both fields are OMITTED when unset rather than sent empty. Absence already
means "cannot say" — an image predating the field says exactly that by not
having the key — so a second spelling would make every reader special-case
it. The pre-existing test asserting the body is EXACTLY {"status": "ok"} is
what keeps a well-meaning `or ""` default from creeping in.

FC_CHANNEL now has one definition. It was read from the environment in
extension.py and would have been read again here; the new build_info module
holds both, and extension.py binds it as a module-level name so existing
tests monkeypatch it exactly as before. Separate from config.py on purpose:
those are operator settings meant to be changed, these describe the artifact.

Channel sits beside the version, never inside it (rule 149), asserted from
both ends. A `-dev` suffix would read as a 0 segment to the extension's
parseInt comparator and make every dev build compare equal — #2993 exactly.

Not hidden, per the operator and §7: the JS bundle and asset hashes
fingerprint the build anyway, and "I'm on 2026.08.28.1249" is the single most
useful line in a bug report.

backend/app/api/extension.py

@@ -7,13 +7,13 @@ from __future__ import annotations
 import asyncio
 import hashlib
 import hmac
-import os
 import re
 from pathlib import Path
 
 from quart import Blueprint, jsonify, request
 from sqlalchemy import select
 
+from ..build_info import FC_CHANNEL as _FC_CHANNEL
 from ..extensions import get_session
 from ..models import AppSetting
 from ..services.extension_service import (
@@ -33,10 +33,13 @@ XPI_DIR = Path("/app/frontend/dist/extension")
 _XPI_VERSION_RE = re.compile(r"fabledcurator-(?P<version>[\w.-]+)\.xpi$")
 
 # Which channel this image belongs to — "dev" or "main" — baked in at build
-# time from the FC_CHANNEL build arg (milestone 271 step 7). Empty for a local
-# build, or for any image predating the field. Tests override by monkeypatching
-# this constant, same as XPI_DIR above.
-FC_CHANNEL = os.environ.get("FC_CHANNEL", "").strip()
+# time (milestone 271 step 7). Read from build_info rather than the environment
+# a second time: /api/health reports the same value, and two independent
+# `os.environ.get` calls are two things that can drift.
+#
+# Still bound as a module-level name here, so tests monkeypatch
+# `extension.FC_CHANNEL` exactly as they did before, same as XPI_DIR above.
+FC_CHANNEL = _FC_CHANNEL
 
 
 async def _ext_key_required(session) -> bool: