Update golangci-lint to v2.12.2 with canonical config

Bump the golangci-lint Docker image pin in Dockerfile and the
release-archive sha256 pins in script/bootstrap from 2.11.3 to
2.12.2, and replace .golangci.yml with the canonical config. The
canonical config moves lll/funlen/cyclop/dupl settings from the
top-level linters-settings key (ignored by the v2 schema) to
linters.settings, so those thresholds now actually apply.

Fix all findings the newly applied thresholds surfaced:

- lll: wrap or shorten seven over-length lines (struct tag
  comments, test logger construction, a func signature, and a
  nosec comment)
- goconst: use http.MethodPost/http.MethodPut and new shared
  constants for repeated test strings; add tmplKeyError and
  tmplKeyWebhook constants for template data keys in handlers
- dupl: merge buildHTTPTargetConfig and buildSlackTargetConfig
  into a parameterized buildURLTargetConfig; drop the duplicate
  iWebhookDB test helper in favor of testWebhookDB; extract
  shared helpers in middleware and session tests

internal/database/model_webhook.go

@@ -4,10 +4,12 @@ package database
 type Webhook struct {
 	BaseModel
 
-	UserID        string `gorm:"type:uuid;not null" json:"userId"`
-	Name          string `gorm:"not null"           json:"name"`
-	Description   string `json:"description"`
-	RetentionDays int    `gorm:"default:30"         json:"retentionDays"` // Days to retain events
+	UserID      string `gorm:"type:uuid;not null" json:"userId"`
+	Name        string `gorm:"not null"           json:"name"`
+	Description string `json:"description"`
+
+	// RetentionDays is the number of days to retain events.
+	RetentionDays int `gorm:"default:30" json:"retentionDays"`
 
 	// Relations
 	User        User         `json:"user,omitzero"`