feat(auth): trust the k8s cluster OIDC issuer for ServiceAccount tokens (ADR-0011 D1)

Additive: gitea-mcp now validates JWTs from a LIST of issuers — the existing
Authentik issuer (DEX_ISSUER_URL) AND, when K8S_ISSUER_URL is set, the in-cluster
k8s OIDC issuer for audience-bound ServiceAccount tokens. Lets in-cluster pods
authenticate with kubelet-rotated projected SA tokens instead of a static bearer.

- config: K8S_ISSUER_URL + K8S_MCP_AUDIENCE.
- cmd/gitea-mcp/k8soidc.go: HTTP client that fetches the k8s OIDC discovery/JWKS
  with the cluster CA + this pod's SA bearer (k3s requires an authed fetch;
  anonymous is 401).
- main.go: build the issuer list; switch NewJWTValidator -> NewMultiJWTValidator.
  The k8s issuer is best-effort — if its client can't be built (not in a pod) or
  its discovery is unreachable at startup, it is DROPPED and we fall back so
  Authentik/static auth is never taken down. Smoke-tested: off-pod it logs the
  skip and starts static-only; static-bearer /mcp returns 400 (auth passed), not 401.
- bump mcp-chassis v0.3.0 -> v0.5.0 (multi-issuer + per-issuer HTTPClient).

Refs infra ADR-0011; enables retiring the in-cluster static bearer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.sum

@@ -1,5 +1,5 @@
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+git.d-ma.be/mathias/mcp-chassis v0.5.0 h1:0w3dt4t4r8OtZBHIOoZkR6p6L3L6YDiqhMh9bTI/w/8=
+git.d-ma.be/mathias/mcp-chassis v0.5.0/go.mod h1:Ks7EK2UnGAN0H3rJjKUxUagX8/ZBdtLrOlcUbv0RwH8=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=