fix: support ES256K OIDC token verification

[Xavrir]

What changed

• Add a narrow ES256K/secp256k1 verifier path for OIDC ID tokens.
• Keep the existing go-oidc/go-jose verification path unchanged for non-ES256K providers.
• Verify ES256K access token hashes with the SHA-256 at_hash calculation.
• Require provider metadata to advertise ES256K unless it is explicitly allowed through SupportedSigningAlgs.
• Add focused tests for valid ES256K tokens, bad signatures, unknown kid values, invalid curve points, missing algorithm metadata, and invalid at_hash values.

Fixes #2534

Testing

• go test ./internal/api/provider -count=1
• go test ./internal/api/... -run 'Test(ParseIDTokenES256K|ES256KRemoteKeySet)' -count=1
• git diff --check

I also attempted the broader local make test path against a temporary Postgres test database. It ran most packages but failed in internal/mailer/validateclient TestValidateEmailExtended because a DNS-dependent invalid-host assertion returned nil locally; that failure is unrelated to this OIDC provider change.