feat(auth): every accepted request names the path that granted it

Closes #60. Closes #58 (fixed in mcp-chassis v0.7.0, consumed here).

Three auth paths reach this server and none of them said so on success.

PassthroughMiddleware logged nothing at all, success or failure. While verifying
#59 end to end there was no way to confirm from the logs whether calls were
using the new PAT pass-through — the caller's own Gitea PAT, the caller's own
identity — or falling through to the shared JWT/static path. The only available
reasoning was inference from which failures were absent.

    INFO mcp auth accepted auth_path=passthrough caller=alice token_fp=...
    INFO mcp auth accepted auth_path=jwt        caller=mathiasbq token_fp=...
    INFO mcp auth accepted auth_path=static     token_fp=...

The first line matters more than the other two. Pass-through is the path where a
request acts as a SPECIFIC Gitea user; an audit trail that cannot say which
identity performed a write is not an audit trail.

chassisauth.Fingerprint, not a local equivalent. The chassis emits token_fp on
its own accepted and rejected lines, and two different fingerprints for the same
token cannot be correlated — which is the only reason the field exists. Exported
from the chassis in v0.7.1 for exactly this.

A token that is not a valid PAT still falls through SILENTLY. The chassis owns
the verdict, and logging here as well would double-count every static-token
request, since all of them reach this middleware first. Asserted, because "logs
nothing on the failure path" is indistinguishable from "was never called".

Also lands #58 by consuming mcp-chassis v0.7.0: a JWT presented to a server with
no JWT auth configured now reports jwt_auth_not_configured rather than
static_token_mismatch. That log line is what sent a debugging session into
re-running the OAuth flow while the credentials were valid the whole time.

Verified: task check EXIT=0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/auth/passthrough.go

@@ -2,9 +2,12 @@ package auth
 
 import (
 	"context"
+	"log/slog"
 	"net/http"
 	"strings"
 
+	chassisauth "git.d-ma.be/mathias/mcp-chassis/auth"
+
 	"git.d-ma.be/mathias/gitea-mcp/internal/gitea"
 )
 
@@ -25,11 +28,34 @@ func PassthroughMiddleware(validator TokenValidator, onValid, fallback http.Hand
 		token, hasBearer := strings.CutPrefix(authz, "Bearer ")
 		if hasBearer && token != "" {
 			if login, ok := validator.ValidateToken(r.Context(), token); ok {
+				// gitea-mcp#60. This middleware logged nothing at all, so there
+				// was no way to tell a PAT pass-through request from one that
+				// fell through to the shared JWT/static path — only inference
+				// from which failures were absent.
+				//
+				// It matters more here than on the other two paths: this is
+				// where a request acts as a SPECIFIC Gitea user. An audit trail
+				// that cannot say which identity performed a write is not an
+				// audit trail.
+				//
+				// chassisauth.Fingerprint, not a local equivalent: the chassis
+				// emits the same field on its own accepted/rejected lines, and
+				// two different fingerprints for one token cannot be correlated.
+				slog.Info("mcp auth accepted",
+					"auth_path", "passthrough",
+					"caller", login,
+					"token_fp", chassisauth.Fingerprint(token),
+					"path", r.URL.Path,
+				)
 				ctx := withCaller(r.Context(), login)
 				ctx = gitea.WithToken(ctx, token)
 				onValid.ServeHTTP(w, r.WithContext(ctx))
 				return
 			}
+			// A token that is not a valid PAT falls through SILENTLY. The
+			// chassis owns the verdict; logging here too would double-count
+			// every static-token request, since all of them reach this
+			// middleware first.
 		}
 		fallback.ServeHTTP(w, r)
 	})