Add OIDC login via Authentik: PKCE, JIT provisioning, email linking (#5)

OIDC is pansy's primary login path (Authentik the target IdP); local
auth (#4) remains the fallback and both issue the same session cookie.

- deps: github.com/coreos/go-oidc/v3 + golang.org/x/oauth2 (both pure Go;
  CGO stays off).
- api/oidc.go: lazy issuer discovery (retried per-request, never crashes
  a server that also serves local auth), GET /auth/oidc/login builds an
  authorization-code URL with PKCE S256 + random state + nonce stashed in
  a short-lived HttpOnly cookie, GET /auth/oidc/callback verifies state
  (constant-time), exchanges the code with the PKCE verifier, verifies the
  ID token + nonce, and starts a pansy session. Failures redirect to
  /login?error=... ; success to /gardens.
- service.LoginOIDC: (issuer,subject) match -> login; else *verified*
  email match -> link onto the existing account; else JIT-create (the IdP
  gates access, so PANSY_REGISTRATION doesn't apply). Unverified email
  colliding with an existing account is refused (takeover guard); no email
  is refused (email is the account key). Reuses the atomic CreateUser.
- store: GetUserByOIDC + LinkOIDC (unique-pair backstop).
- config: OIDCReady() (needs issuer+client+BaseURL for the redirect URI);
  /auth/providers now reports oidc from it and defaults the button label
  to "Sign in with Authentik". OIDC routes are only registered when ready,
  so an unconfigured instance 404s them.
- PANSY_LOCAL_AUTH=false rejects/hides local auth but not OIDC.

Tests: service provisioning (JIT, repeat login, link, unverified-collision
refusal, no-email, name fallback, works with local auth off); api
(routes-absent-when-unconfigured, providers reporting, login redirect with
PKCE params + tx cookie via a fake discovery server, callback state/error
paths). Smoke-tested: unreachable issuer degrades to error=oidc_unavailable
with the server still up and local auth working.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

go.sum

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@@ -94,6 +98,8 @@ golang.org/x/mod v0.17.0 h1:zY54UmvipHiNd+pm+m0x9KhZ9hl1/7QNMyxXbc6ICqA=
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+golang.org/x/oauth2 v0.36.0 h1:peZ/1z27fi9hUOFCAZaHyrpWG5lwe0RJEEEeH0ThlIs=
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