Add Gitea SSH setup script and integrate Gitea into partnership/failover

Partnership/gitea_ssh_setup.sh: generates ed25519 keypair, registers public key
  with Gitea API (POST /api/v1/user/keys), tests SSH connection, writes ~/.ssh/config
  entry. Idempotent — skips steps already done. --force regenerates and re-registers.
  Resolves Gitea endpoint from container IP first, falls back to GITEA_DOMAIN.

common.sh: alias HOST*_GITEA_API_TOKEN → GITEA_API_TOKEN in detect_hosts().
  Add to detect_hosts() doc comment.

master.conf: add GITEA_HTTP_PORT=3000 for API endpoint construction.

host1.conf: add HOST1_GITEA_API_TOKEN (fill in from Gitea Settings → Applications).
  Add my-Gitea.xml to HOST1_PARTNERSHIP_AUTH_STACK — onboard pushes it to HOST2.

host2.conf: add Gitea to FALLBACK_HOST2_COVERS_HOST1_TIER1 — starts immediately
  when HOST1 goes down, making the source of truth reachable independently of
  HOST1's auth stack.

host2.conf

@@ -337,6 +337,7 @@
 # Higher tiers activate after HOST1_TIER*_DELAY minutes (set in host1.conf).
     FALLBACK_HOST2_COVERS_HOST1_TIER1=(
         "Gmer4Lfe.com"
+        "Gitea"                 # source of truth — must be reachable even when HOST1 auth stack is down
         "Emby"
         "VaultWarden-Gmer4Lfe"
         "Dispatcharr"