refactor: use pinned golangci-lint Docker image for linting

Refactor Dockerfile to use a separate lint stage with a pinned
golangci-lint v2.11.3 Docker image instead of installing
golangci-lint via curl in the builder stage. This follows the
pattern used by sneak/pixa.

Changes:
- Dockerfile: separate lint stage using golangci/golangci-lint:v2.11.3
  (Debian-based, pinned by sha256) with COPY --from=lint dependency
- Bump Go from 1.24 to 1.26.1 (golang:1.26.1-bookworm, pinned)
- Bump golangci-lint from v1.64.8 to v2.11.3
- Migrate .golangci.yml from v1 to v2 format (same linters, format only)
- All Docker images pinned by sha256 digest
- Fix all lint issues from the v2 linter upgrade:
  - Add package comments to all packages
  - Add doc comments to all exported types, functions, and methods
  - Fix unchecked errors (errcheck)
  - Fix unused parameters (revive)
  - Fix gosec warnings (MaxBytesReader for form parsing)
  - Fix staticcheck suggestions (fmt.Fprintf instead of WriteString)
  - Rename DeliveryTask to Task to avoid stutter (delivery.Task)
  - Rename shadowed builtin 'max' parameter
- Update README.md version requirements

internal/database/model_delivery.go

@@ -3,6 +3,7 @@ package database
 // DeliveryStatus represents the status of a delivery
 type DeliveryStatus string
 
+// Delivery status values.
 const (
 	DeliveryStatusPending   DeliveryStatus = "pending"
 	DeliveryStatusDelivered DeliveryStatus = "delivered"
@@ -14,12 +15,12 @@ const (
 type Delivery struct {
 	BaseModel
 
-	EventID  string         `gorm:"type:uuid;not null" json:"event_id"`
-	TargetID string         `gorm:"type:uuid;not null" json:"target_id"`
+	EventID  string         `gorm:"type:uuid;not null"         json:"eventId"`
+	TargetID string         `gorm:"type:uuid;not null"         json:"targetId"`
 	Status   DeliveryStatus `gorm:"not null;default:'pending'" json:"status"`
 
 	// Relations
-	Event           Event            `json:"event,omitempty"`
-	Target          Target           `json:"target,omitempty"`
-	DeliveryResults []DeliveryResult `json:"delivery_results,omitempty"`
+	Event           Event            `json:"event,omitzero"`
+	Target          Target           `json:"target,omitzero"`
+	DeliveryResults []DeliveryResult `json:"deliveryResults,omitempty"`
 }