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_target.go

@@ -3,6 +3,7 @@ package database
 // TargetType represents the type of delivery target
 type TargetType string
 
+// Target type values.
 const (
 	TargetTypeHTTP     TargetType = "http"
 	TargetTypeDatabase TargetType = "database"
@@ -14,19 +15,19 @@ const (
 type Target struct {
 	BaseModel
 
-	WebhookID string     `gorm:"type:uuid;not null" json:"webhook_id"`
-	Name      string     `gorm:"not null" json:"name"`
-	Type      TargetType `gorm:"not null" json:"type"`
-	Active    bool       `gorm:"default:true" json:"active"`
+	WebhookID string     `gorm:"type:uuid;not null" json:"webhookId"`
+	Name      string     `gorm:"not null"           json:"name"`
+	Type      TargetType `gorm:"not null"           json:"type"`
+	Active    bool       `gorm:"default:true"       json:"active"`
 
 	// Configuration fields (JSON stored based on type)
 	Config string `gorm:"type:text" json:"config"` // JSON configuration
 
 	// For HTTP targets (max_retries=0 means fire-and-forget, >0 enables retries with backoff)
-	MaxRetries   int `json:"max_retries,omitempty"`
-	MaxQueueSize int `json:"max_queue_size,omitempty"`
+	MaxRetries   int `json:"maxRetries,omitempty"`
+	MaxQueueSize int `json:"maxQueueSize,omitempty"`
 
 	// Relations
-	Webhook    Webhook    `json:"webhook,omitempty"`
+	Webhook    Webhook    `json:"webhook,omitzero"`
 	Deliveries []Delivery `json:"deliveries,omitempty"`
 }