Enforce and document exact-match-only for signature verification (#40)

Closes #27

Signatures are per-URL only — this PR adds explicit tests and documentation enforcing that HMAC-SHA256 signatures verify against exact URLs only. No suffix matching, wildcard matching, or partial matching is supported.

## What this does NOT touch

**The host whitelist code (`whitelist.go`) is not modified.** This PR is exclusively about signature verification, per sneak's instructions on [issue #27](#27), [PR #32](#32), and [PR #35](#35).

## Changes

### `internal/imgcache/signature.go`
- Added documentation comments on `Verify()` and `buildSignatureData()` explicitly specifying that signatures are exact-match only — no suffix, wildcard, or partial matching

### `internal/imgcache/signature_test.go`
- **`TestSigner_Verify_ExactMatchOnly`**: 14 tamper cases verifying that modifying any signed component (host, path, query, dimensions, format) causes verification to fail. Host-specific cases include:
  - Parent domain (`example.com`) does not match subdomain signature (`cdn.example.com`)
  - Sibling subdomain (`images.example.com`) does not match
  - Deeper subdomain (`images.cdn.example.com`) does not match
  - Evil suffix domain (`cdn.example.com.evil.com`) does not match
  - Prefixed host (`evilcdn.example.com`) does not match
- **`TestSigner_Sign_ExactHostInData`**: Verifies that suffix-related hosts (`cdn.example.com`, `example.com`, `images.example.com`, etc.) all produce distinct signatures

### `internal/imgcache/service_test.go`
- **`TestService_ValidateRequest_SignatureExactHostMatch`**: Integration test through `ValidateRequest` verifying that a valid signature for `cdn.example.com` is rejected when presented with a different host (parent domain, sibling subdomain, deeper subdomain, evil suffix, prefixed host)

### `README.md`
- Updated Signature Specification section to explicitly document exact-match-only semantics

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #40
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/imgcache/service_test.go

@@ -151,6 +151,74 @@ func TestService_Get_NonWhitelistedHost_InvalidSignature(t *testing.T) {
 	}
 }
 
+// TestService_ValidateRequest_SignatureExactHostMatch verifies that
+// ValidateRequest enforces exact host matching for signatures. A
+// signature for one host must not verify for a different host, even
+// if they share a domain suffix.
+func TestService_ValidateRequest_SignatureExactHostMatch(t *testing.T) {
+	signingKey := "test-signing-key-must-be-32-chars"
+	svc, _ := SetupTestService(t,
+		WithSigningKey(signingKey),
+		WithNoWhitelist(),
+	)
+
+	signer := NewSigner(signingKey)
+
+	// Sign a request for "cdn.example.com"
+	signedReq := &ImageRequest{
+		SourceHost: "cdn.example.com",
+		SourcePath: "/photos/cat.jpg",
+		Size:       Size{Width: 50, Height: 50},
+		Format:     FormatJPEG,
+		Quality:    85,
+		FitMode:    FitCover,
+		Expires:    time.Now().Add(time.Hour),
+	}
+	signedReq.Signature = signer.Sign(signedReq)
+
+	// The original request should pass validation
+	t.Run("exact host passes", func(t *testing.T) {
+		err := svc.ValidateRequest(signedReq)
+		if err != nil {
+			t.Errorf("ValidateRequest() exact host failed: %v", err)
+		}
+	})
+
+	// Try to reuse the signature with different hosts
+	tests := []struct {
+		name string
+		host string
+	}{
+		{"parent domain", "example.com"},
+		{"sibling subdomain", "images.example.com"},
+		{"deeper subdomain", "a.cdn.example.com"},
+		{"evil suffix domain", "cdn.example.com.evil.com"},
+		{"prefixed host", "evilcdn.example.com"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name+" rejected", func(t *testing.T) {
+			req := &ImageRequest{
+				SourceHost:  tt.host,
+				SourcePath:  signedReq.SourcePath,
+				SourceQuery: signedReq.SourceQuery,
+				Size:        signedReq.Size,
+				Format:      signedReq.Format,
+				Quality:     signedReq.Quality,
+				FitMode:     signedReq.FitMode,
+				Expires:     signedReq.Expires,
+				Signature:   signedReq.Signature,
+			}
+
+			err := svc.ValidateRequest(req)
+			if err == nil {
+				t.Errorf("ValidateRequest() should reject signature for host %q (signed for %q)",
+					tt.host, signedReq.SourceHost)
+			}
+		})
+	}
+}
+
 func TestService_Get_InvalidFile(t *testing.T) {
 	svc, fixtures := SetupTestService(t)
 	ctx := context.Background()