Add webhook logging, pagination, middleware, migrations, and prod hardening

- Webhook event logging repo and subscription webhook idempotency
- Pagination helper (echohelpers) with cursor/offset support
- Request ID and structured logging middleware
- Push client improvements (FCM HTTP v1, better error handling)
- Task model version column, business constraint migrations, targeted indexes
- Expanded categorization chain tests
- Email service and config hardening
- CI workflow updates, .gitignore additions, .env.example updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/echohelpers/pagination.go

@@ -0,0 +1,32 @@
+package echohelpers
+
+import (
+	"strconv"
+
+	"github.com/labstack/echo/v4"
+)
+
+// ParsePagination extracts limit and offset from query parameters with bounded defaults.
+// maxLimit caps the maximum page size to prevent unbounded queries.
+func ParsePagination(c echo.Context, maxLimit int) (limit, offset int) {
+	const defaultLimit = 50
+
+	limit = defaultLimit
+	if l := c.QueryParam("limit"); l != "" {
+		if parsed, err := strconv.Atoi(l); err == nil && parsed > 0 {
+			limit = parsed
+		}
+	}
+	if limit > maxLimit {
+		limit = maxLimit
+	}
+
+	offset = 0
+	if o := c.QueryParam("offset"); o != "" {
+		if parsed, err := strconv.Atoi(o); err == nil && parsed >= 0 {
+			offset = parsed
+		}
+	}
+
+	return limit, offset
+}