feat(auth): adopt chassis v0.3.0 — auth audit logs + 503 on Dex outage (#6, #9)

Bumps mcp-chassis to v0.3.0, which adds structured audit logging on every auth
rejection and returns 503 temporarily_unavailable (not a silent 401) when Dex is
unreachable at validation time. Wires slog.SetDefault so those audit lines flow
through gitea-mcp's JSON handler.

Together with the earlier /healthz jwt-status reporting and startup degradation
warning, this closes #6 (Dex-down is now observable and distinct from a bad
token) and #9 (auth failures are audit-logged: reason, IP, token type, hashed
fingerprint — never the raw token).

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 v0.3.0 h1:lV/vDsjrDeZojT7lhcwolM1lMZpsnEKEvf4kEHrxIa0=
+git.d-ma.be/mathias/mcp-chassis v0.3.0/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=