feat(web): implement Dex OIDC auth (web.Auth) with single-user allowlist

Adds internal/web/oidc.DexAuth, the production web.Auth impl behind the seam
(ADR-011, docs/ui-spec.md §6). Standard Authorization Code flow against Dex:

- Routes() mounts /auth/login (state+nonce, redirect to authorize),
  /auth/callback (code exchange, ID-token verify, nonce check, allowlist:
  sub must equal Config.AllowedSubject else 403, set session, redirect /),
  /auth/logout (clear session).
- Middleware redirects unauthenticated requests to /auth/login, slides the
  session expiry on each authenticated request; /healthz and /auth/* bypass.
- CurrentUser resolves the principal from the session cookie.
- Sessions: server-side in-memory store (single Stage-0 replica) keyed by an
  HMAC-SHA256 (HS256) signed, HttpOnly, Secure, SameSite=Lax cookie with a
  short TTL + sliding refresh. State->nonce pending map is one-time + expiring
  (replay/CSRF defense). Tokens are never logged.

Constructor New(ctx, Config, ...Option); the six-field Config (Issuer,
ClientID, ClientSecret, RedirectURL, SessionSecret, AllowedSubject) is what
cmd/tapir wires from TAPIR_OIDC_*/TAPIR_DEX_*/TAPIR_SESSION_SECRET/
TAPIR_ALLOWED_SUBJECT. Options (clock, TTL, insecure cookies) are test-only.

Tests use a fake OIDC issuer via httptest (discovery + JWKS + token endpoint
signing an RS256 ID token) — no live Dex: login 302s to authorize; callback
for the allowlisted sub sets a session and 302s to /; non-allowlisted sub 403;
middleware redirects unauthenticated and passes authenticated; logout clears;
plus expiry, tampered-cookie, and unknown-state cases.

Deps (per ADR-006 / ui-spec §6): adds github.com/coreos/go-oidc/v3 — the
homelab-standard OIDC lib, small, handles discovery + JWKS + ID-token
verification; pairs with the already-present golang.org/x/oauth2. go-jose/v4
(transitive via go-oidc) is used directly only in tests to sign the fake
issuer's tokens.

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

go.sum

@@ -6,6 +6,8 @@ github.com/containerd/errdefs v1.0.0 h1:tg5yIfIlQIrxYtu9ajqY42W3lpS19XqdxRQeEwYG
 github.com/containerd/errdefs v1.0.0/go.mod h1:+YBYIdtsnF4Iw6nWZhJcqGSg/dwvV7tyJ/kCkyJ2k+M=
 github.com/containerd/errdefs/pkg v0.3.0 h1:9IKJ06FvyNlexW690DXuQNx2KA2cUJXx151Xdx3ZPPE=
 github.com/containerd/errdefs/pkg v0.3.0/go.mod h1:NJw6s9HwNuRhnjJhM7pylWwMyAkmCQvQ4GpJHEqRLVk=
+github.com/coreos/go-oidc/v3 v3.18.0 h1:V9orjXynvu5wiC9SemFTWnG4F45v403aIcjWo0d41+A=
+github.com/coreos/go-oidc/v3 v3.18.0/go.mod h1:DYCf24+ncYi+XkIH97GY1+dqoRlbaSI26KVTCI9SrY4=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1VwoXQT9A3Wy9MM3WgvqSxFWenqJduM=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -23,6 +25,8 @@ github.com/felixge/httpsnoop v1.0.4 h1:NFTV2Zj1bL4mc9sqWACXbQFVBBg2W3GPvqp8/ESS2
 github.com/felixge/httpsnoop v1.0.4/go.mod h1:m8KPJKqk1gH5J9DgRY2ASl2lWCfGKXixSwevea8zH2U=
 github.com/fergusstrange/embedded-postgres v1.34.0 h1:c6RKhPKFsLVU+Tdxsx8q0UxCHsvZZ/iShAnljRBXs6s=
 github.com/fergusstrange/embedded-postgres v1.34.0/go.mod h1:w0YvnCgf19o6tskInrOOACtnqfVlOvluz3hlNLY7tRk=
+github.com/go-jose/go-jose/v4 v4.1.4 h1:moDMcTHmvE6Groj34emNPLs/qtYXRVcd6S7NHbHz3kA=
+github.com/go-jose/go-jose/v4 v4.1.4/go.mod h1:x4oUasVrzR7071A4TnHLGSPpNOm2a21K9Kf04k1rs08=
 github.com/go-logr/logr v1.4.3 h1:CjnDlHq8ikf6E492q6eKboGOC0T8CDaOvkHCIg8idEI=
 github.com/go-logr/logr v1.4.3/go.mod h1:9T104GzyrTigFIr8wt5mBrctHMim0Nb2HLGrmQ40KvY=
 github.com/go-logr/stdr v1.2.2 h1:hSWxHoqTgW2S2qGc0LTAI563KZ5YKYRhT3MFKZMbjag=
@@ -83,8 +87,8 @@ go.opentelemetry.io/otel/trace v1.37.0 h1:HLdcFNbRQBE2imdSEgm/kwqmQj1Or1l/7bW6mx
 go.opentelemetry.io/otel/trace v1.37.0/go.mod h1:TlgrlQ+PtQO5XFerSPUYG0JSgGyryXewPGyayAWSBS0=
 go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
 go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
-golang.org/x/oauth2 v0.30.0 h1:dnDm7JmhM45NNpd8FDDeLhK6FwqbOf4MLCM9zb1BOHI=
-golang.org/x/oauth2 v0.30.0/go.mod h1:B++QgG3ZKulg6sRPGD/mqlHQs5rB3Ml9erfeDY7xKlU=
+golang.org/x/oauth2 v0.36.0 h1:peZ/1z27fi9hUOFCAZaHyrpWG5lwe0RJEEEeH0ThlIs=
+golang.org/x/oauth2 v0.36.0/go.mod h1:YDBUJMTkDnJS+A4BP4eZBjCqtokkg1hODuPjwiGPO7Q=
 golang.org/x/sync v0.18.0 h1:kr88TuHDroi+UVf+0hZnirlk8o8T+4MrK6mr60WkH/I=
 golang.org/x/sync v0.18.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=
 golang.org/x/sys v0.38.0 h1:3yZWxaJjBmCWXqhN1qh02AkOnCQ1poK6oF+a7xWL6Gc=