Rate-limit the public webhook receiver endpoint (closes #64)

The public receiver /webhook/{uuid} had no rate limiting: anyone
who learns an entrypoint UUID can flood it, inflating the
per-webhook database and the delivery queue.

Add a dedicated limit scoped to the receiver route, keyed per
client IP per request path (the path contains the entrypoint
UUID), so one misbehaving sender is throttled without affecting
other senders of the same entrypoint or other entrypoints.
Requests over the limit get a 429; httprate adds the Retry-After
header per RFC 6585. IP extraction honours X-Forwarded-For,
X-Real-IP, and True-Client-IP for reverse-proxy deployments.

The limit is RECEIVER_RATE_LIMIT requests per minute, default
120. A set-but-unparseable or non-positive value aborts startup
via the new envPositiveInt strict parser rather than silently
falling back to the default. envInt's other callers are
unchanged; converting them is tracked in #80.

Also update the README env table and Rate Limiting design
section, and sync TODO.md.

internal/config/config.go

@@ -175,27 +175,35 @@ func envDuration(
 	return d, nil
 }
 
+// resolveEnvironment reads WEBHOOKER_ENVIRONMENT, defaulting to
+// dev, and rejects unrecognised values.
+func resolveEnvironment() (string, error) {
+	environment := os.Getenv("WEBHOOKER_ENVIRONMENT")
+	if environment == "" {
+		environment = EnvironmentDev
+	}
+
+	if environment != EnvironmentDev &&
+		environment != EnvironmentProd {
+		return "", fmt.Errorf(
+			"%w: WEBHOOKER_ENVIRONMENT must be '%s' or '%s', got '%s'",
+			ErrInvalidEnvironment,
+			EnvironmentDev, EnvironmentProd, environment,
+		)
+	}
+
+	return environment, nil
+}
+
 // New creates a Config by reading environment variables.
 //
 //nolint:revive // lc parameter is required by fx even if unused.
 func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 	log := params.Logger.Get()
 
-	// Determine environment from WEBHOOKER_ENVIRONMENT env var,
-	// default to dev
-	environment := os.Getenv("WEBHOOKER_ENVIRONMENT")
-	if environment == "" {
-		environment = EnvironmentDev
-	}
-
-	// Validate environment
-	if environment != EnvironmentDev &&
-		environment != EnvironmentProd {
-		return nil, fmt.Errorf(
-			"%w: WEBHOOKER_ENVIRONMENT must be '%s' or '%s', got '%s'",
-			ErrInvalidEnvironment,
-			EnvironmentDev, EnvironmentProd, environment,
-		)
+	environment, err := resolveEnvironment()
+	if err != nil {
+		return nil, err
 	}
 
 	// Parse the retention sweep interval; a set-but-unparseable value