feat: wire the Instagram connector into connect and publishing

The Instagram connector existed but was never registered, so Channels showed
it as "provider unavailable" and publishing could not resolve it.

- Register createInstagramConnector in the connections registry and the
  publishing registry; add the connector-runtime publishing policy
  (idempotency "none": media_publish has no key, never replay).
- The Facebook app credentials identify the Hub, not a collective: read them
  only from INSTAGRAM_CLIENT_ID / INSTAGRAM_CLIENT_SECRET. Drop the
  clientId/clientSecret connect fields and stop copying the app secret into
  each connection's vault entry. The provider is connectable only when both
  are set (connectEnabled + connectNote).
- Fail the connect when several Instagram accounts are granted instead of
  silently publishing as the first Page's account.
- Fix the connector package's pre-existing lint errors (it now ships in api).
- Tests: contract cases for env-only credentials and the multi-account
  refusal; connections integration case for provider gating and the
  Facebook dialog redirect.
- docs/oauth-connect.md: Instagram setup, callback URI and limits.

Not in this change: publishing images needs a public plaintext JPEG URL that
Meta downloads, and storage is client-side encrypted, so presigned S3 URLs
cannot serve it. Until a short-lived media endpoint exists, publishing to
Instagram fails with an explicit error. Token refresh via ig_refresh_token
on graph.facebook.com also needs checking against Facebook Login tokens.

apps/api/package.json

@@ -13,6 +13,7 @@
   "dependencies": {
     "@tactical-media-broadcast-hub/connector-sdk": "workspace:*",
     "@tactical-media-broadcast-hub/connector-telegram": "workspace:*",
+    "@tactical-media-broadcast-hub/connector-instagram": "workspace:*",
     "@tactical-media-broadcast-hub/core": "workspace:*",
     "@aws-sdk/client-s3": "^3.1146.0",
     "@aws-sdk/s3-request-presigner": "^3.1146.0",