feat(gotify): put the Gotify UI behind ZITADEL single sign-on

Gotify has no OIDC support, so it cannot validate a ZITADEL token itself. Put
oauth2-proxy in front of it and let nginx ask the proxy for every request.

The Gotify REST API keeps its own client-token check. /message, /application,
/client, /user, /version, /stream and /health are skipped by the proxy, so
Forgejo webhooks, the harness, poimen-sender and the Android client keep working
without a browser session. Only the web UI requires SSO.

The /oauth2 path needs its own ingress: the auth_request annotation would
otherwise also guard /oauth2/start, and the login entry point would demand a
login first.

The client id and secret come from project role provisioning:
`zitadel-provision.py apply` now creates the `gotify` OIDC app, and the secret is
SOPS-encrypted under k8s/argocd/secrets.

Gotify's own login remains behind the proxy. ZITADEL decides who reaches the
service, Gotify decides what that person may do inside it.

k8s/argocd/secrets/gotify-oauth2-proxy-secrets.enc.yaml

@@ -0,0 +1,30 @@
+apiVersion: v1
+kind: Secret
+metadata:
+    name: gotify-oauth2-proxy
+    namespace: notifications
+type: Opaque
+data:
+    client-id: ENC[AES256_GCM,data:Q+XQAR6cfHKoiuxhPZs6khTvqT0A/JG6,iv:LqhfuhXOYPVB6FQpPpERzWRQ+nxgxHZo4qdJO/g52y8=,tag:V2eK2fyscQKmM3/MmDnNtg==,type:str]
+    client-secret: ENC[AES256_GCM,data:P7F+83CLDhHnTN+Qx3SeCFbCjVH4JHb2rSteCr3Dywcb1ysMSNvC+3HyBXzsfJJ/EErUrCPWuwlRmV5ySts1oIrWhKyWMJxNrUTzv4eoclat4RyV6SaNDg==,iv:ESO1OUIqZbZPEVUuHrqjskA4tuTX/sc5m+K6Eo319kI=,tag:2isdxjFbtosov88FHZdeaQ==,type:str]
+    cookie-secret: ENC[AES256_GCM,data:WoWiANSdju+Ep5MuZQNeT0BQdKf5cFvrDUZpe+4B0VeUmQqnvblFi3dciJOeBFom9xYYzUCAI2tUSmX+a6tdAObyoGvjdFmLh+WTfw==,iv:41gYYRkyDIqC3GPuZkoob81Wl2njFTY07j4Puzaq0s4=,tag:x6z33zZnFwvmWUQIVPcxdQ==,type:str]
+sops:
+    kms: []
+    gcp_kms: []
+    azure_kv: []
+    hc_vault: []
+    age:
+        - recipient: age1e5fq3hwxy78psus2nfvmtmua36g0u3suk78ephw6246l974d2utsvn0hla
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSB2UURWblY4aTBOQjFSNDRL
+            YkRXVjVHK1QvYW1iR2o0UWVmdE94c2lkWDJjCnlXK1k3cnZBNEExbDU3eDhQZ3FT
+            TVcyckFiWHdRSjZHQ201Nnl6UFdTN28KLS0tIExQeE90VnM1WHRCYkpycjJtR0ht
+            ZHFDNWhKeENSQzljZUNmdktDYWloUDQK/fOG8AjTb2TNqLbqNfArOs0mzInYNJVd
+            iaYxnjH6AjhbLKdnmCpecTQpgx8GeFGIkefV/x5pgrdSUwn0lcvpSQ==
+            -----END AGE ENCRYPTED FILE-----
+    lastmodified: "2026-10-27T19:05:35Z"
+    mac: ENC[AES256_GCM,data:KZV3JNebQ+DbZZZ+e3fvtOGckwC9fmkIchh8DBJ0DMq9lbrPEhP4Juordmnrrs9vJvGxuFJ/KwUv9EELqBsuSy8c47+z7flEAXJiJCJkvVPL/jMidABsFqMALvNNrCqBPWvMHLZUj762DwxB2gKhSx3GLfRUXhm+ZiNcD+39EVw=,iv:ifUlEq/GhBc/QomJApkCaWc3f4ggXoLsvqjzZElO0fU=,tag:fMo+HBDUZdRXgfck+8AJbA==,type:str]
+    pgp: []
+    encrypted_regex: ^(data|stringData)$
+    version: 3.9.3

k8s/argocd/secrets/secret-generator.yaml

@@ -36,6 +36,7 @@ files:
     - etcd-defrag-creds.enc.yaml
     - zitadel-masterkey.enc.yaml
     - smtp-gotify-secrets.enc.yaml
+    - gotify-oauth2-proxy-secrets.enc.yaml
     # Login UI RSA keypair, adopted from the chart-generated Secret so ArgoCD
     # owns it. Without this, the chart re-generates the keypair on every render,
     # which changes checksum/secret-login-service-key and leaves both Zitadel