gitea: one instance, a token per person

Gitea is the first service where the server is shared but the account is not.
The owner connects the instance; every other user supplies only their own access
token and sees their own repositories, notifications and issues.

The plumbing was already per-user — createSidecarProxy injects the authenticated
caller's id as X-Officer-User, the sidecar refuses a request without it, and
service_connections is UNIQUE (user_id, service). What was wrong is that url and
token lived in the same row and a save demanded both, so a member would have had
to type the instance URL. That is worse than inconvenient: a member who can name
the URL has a per-user SSRF hop behind a settings form, and the sidecar would
dutifully attach their token to it.

`url` is now nullable, and NULL means "inherit the instance". The owner's row
carries the URL and IS the instance; everyone else's row is a credential. A
member's URL is therefore not stored rather than merely hidden — which is what
makes "members never see the instance URL" a property of the schema instead of a
filter somebody has to remember on every response.

Resolution lives in one place (getResolvedServiceCredentials / getServiceInstanceUrl)
rather than in each sidecar, so there is a single answer to "where is this
service" and no sidecar can accidentally trust a member-supplied URL.

Rules, all enforced in the sidecar rather than the UI, because a form that hides
a field is a suggestion and these are rules:

  owner PUT /_config    { url, token }, as before
  member PUT /_config   { token } only; a url in the body is REJECTED, not
                        ignored — ignoring it would leave someone debugging a
                        screen quietly talking to a different server
  member, no instance   409, "the server owner has not connected a Gitea
                        instance yet"
  member GET /_config   has no url to return
  owner disconnects     members keep their tokens and resolve to nothing; no
                        instance, no service

GET /_config also now answers `instanceConfigured` and `isOwner`, which is what
lets the UI tell "you have not connected yet" from "there is nothing here to
connect to" — different screens.

memos, slskd and transmission front a single daemon and always store their own
URL; they now treat a null as a malformed row rather than reaching for somebody
else's instance.

Verified on a scratch database: pushing twice adds no diff churn beyond the known
pk_music_now_playing pair, and the resolution behaves — member GET returns a null
url, member credentials resolve to the owner's base with the member's own token,
and deleting the owner's row leaves the member's token intact but resolving to
nothing. All four live rows have a url today, so the column change applies
without touching data.

Not reachable by a real member yet: the account backstop still confines
non-owners to /api/auth + /api/music. This works the moment the capability model
lands, and until then is testable only by minting a token.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/servers/sidecar/transmission/upstream.ts

@@ -55,7 +55,9 @@ export async function getTransmissionConfig(userId: number): Promise<Transmissio
   if (hit && Date.now() - hit.at < TTL_MS) return hit.cfg;
 
   const creds = await getServiceCredentials(userId, 'transmission');
-  const cfg = creds
+  // `url` became nullable when services that inherit one instance landed (see the schema column note).
+  // Transmission fronts a single daemon and always stores its own, so null here is a malformed row.
+  const cfg = creds?.url
     ? {
         base: normalizeBase(creds.url),
         rpcPath: normalizeRpcPath(creds.path),