feat(auth): JWT-or-static middleware + /.well-known/oauth-protected-resource (issue #5)

- internal/auth/jwt.go: JWTValidator via lestrrat-go/jwx/v2, JWKS auto-refresh
- internal/auth/bearer.go: replace Gitea PAT validation with JWT->static->default chain
- internal/gitea/client.go: always use service PAT; remove TokenFromContext lookup
- internal/config/config.go: add DexIssuerURL, MCPAudience, MCPResourceURL, StaticToken
- cmd/gitea-mcp/main.go: wire validator, fix /.well-known to return real AS list
- bearer_test.go: rewrite for new API

internal/config/config.go

@@ -8,9 +8,13 @@ import (
 type Config struct {
 	Port            string   // GITEA_MCP_PORT, default 8080
 	GiteaBaseURL    string   // GITEA_BASE_URL, e.g. https://gitea.d-ma.be
-	DefaultToken    string   // GITEA_MCP_DEFAULT_TOKEN, fallback PAT when no Bearer header present (e.g. claude.ai)
+	DefaultToken    string   // GITEA_MCP_DEFAULT_TOKEN, service PAT; used by Gitea client for all upstream calls
+	StaticToken     string   // GITEA_MCP_STATIC_TOKEN, optional static bearer for service-to-service auth
 	AllowedOwners   []string // GITEA_MCP_ALLOWED_OWNERS, comma-separated, default "mathias"
 	OriginAllowlist []string // GITEA_MCP_ORIGIN_ALLOWLIST, comma-separated
+	DexIssuerURL    string   // DEX_ISSUER_URL, e.g. https://auth.d-ma.be; empty disables JWT auth
+	MCPAudience     string   // MCP_AUDIENCE, JWT audience claim to validate, e.g. claude-ai
+	MCPResourceURL  string   // MCP_RESOURCE_URL, this server's public URL for /.well-known metadata
 }
 
 func Load() (Config, error) {
@@ -18,8 +22,12 @@ func Load() (Config, error) {
 		Port:            envOr("GITEA_MCP_PORT", "8080"),
 		GiteaBaseURL:    os.Getenv("GITEA_BASE_URL"),
 		DefaultToken:    os.Getenv("GITEA_MCP_DEFAULT_TOKEN"),
+		StaticToken:     os.Getenv("GITEA_MCP_STATIC_TOKEN"),
 		AllowedOwners:   splitCSV(envOr("GITEA_MCP_ALLOWED_OWNERS", "mathias")),
 		OriginAllowlist: splitCSV(os.Getenv("GITEA_MCP_ORIGIN_ALLOWLIST")),
+		DexIssuerURL:    os.Getenv("DEX_ISSUER_URL"),
+		MCPAudience:     os.Getenv("MCP_AUDIENCE"),
+		MCPResourceURL:  os.Getenv("MCP_RESOURCE_URL"),
 	}
 	return cfg, nil
 }