Add optional inbound webhook signature verification (closes #67)

A receiver URL was a bare v4 UUID and nothing else: anyone who learned
it could store events and, because inbound headers are forwarded to
targets almost verbatim, choose what the downstream service received.

Entrypoints gain an optional scheme/secret pair. GitHub's
X-Hub-Signature-256 (HMAC-SHA256 hex over the raw body) and GitLab's
X-Gitlab-Token (plain shared token) are supported; both compare with
hmac.Equal. With nothing configured an entrypoint behaves exactly as
before, which is also where every pre-existing row lands after
AutoMigrate adds the columns.

Verification runs after the capped body read and before the first
write, so a rejected request leaves no event row, no delivery row and
no delivery task. A configuration the receiver cannot apply — unknown
scheme, or one half of the pair missing — is refused with a 500 rather
than falling back to unverified.

The secret is credential-bearing and is stored in the clear because
HMAC needs the key itself. It is excluded from JSON, kept out of
templates by a new handlers.EntrypointView projection, and absent from
every log line including the rejection path. The UI sets and rotates
it through one form that never renders the stored value.

internal/database/model_entrypoint.go

@@ -1,5 +1,22 @@
 package database
 
+// SignatureScheme names the way an entrypoint authenticates inbound
+// requests. A scheme fixes both the header the signature arrives in
+// and the algorithm used to check it, so an operator cannot pair one
+// sender's header with another sender's comparison.
+type SignatureScheme string
+
+// Signature scheme values. The empty scheme means the entrypoint
+// performs no inbound verification: it is the default, and it is the
+// state every entrypoint created before this column existed migrates
+// to, so an existing deployment keeps accepting the requests it
+// accepted before.
+const (
+	SignatureSchemeNone   SignatureScheme = ""
+	SignatureSchemeGitHub SignatureScheme = "github"
+	SignatureSchemeGitLab SignatureScheme = "gitlab"
+)
+
 // Entrypoint represents an inbound URL endpoint that feeds into a webhook
 type Entrypoint struct {
 	BaseModel
@@ -12,6 +29,31 @@ type Entrypoint struct {
 	Description string `json:"description"`
 	Active      bool   `gorm:"default:true" json:"active"`
 
+	// SignatureScheme selects how inbound requests to this
+	// entrypoint are authenticated. Empty means unauthenticated,
+	// which is what a UUID-only entrypoint has always been.
+	SignatureScheme SignatureScheme `gorm:"default:''" json:"signatureScheme"`
+
+	// SignatureSecret is the secret shared with the sender.
+	//
+	// It is stored in the clear because HMAC verification needs the
+	// key itself: a hash of it cannot recompute the sender's digest.
+	// It is therefore a live credential, and json:"-" keeps it out of
+	// any handler that marshals the model, the way APIKey.Key and
+	// Target.Config are kept out. handlers.EntrypointView is the
+	// matching barrier for the HTML path.
+	SignatureSecret string `gorm:"default:''" json:"-"`
+
 	// Relations
 	Webhook Webhook `json:"webhook,omitzero"`
 }
+
+// SignatureConfigured reports whether this entrypoint verifies
+// inbound requests. Both halves must be present: a scheme without a
+// secret, or a secret without a scheme, is a broken configuration
+// rather than a configured one, and signature.Verify fails those
+// closed rather than treating them as "off".
+func (e *Entrypoint) SignatureConfigured() bool {
+	return e.SignatureScheme != SignatureSchemeNone &&
+		e.SignatureSecret != ""
+}