feat: enhance notification system and migrate to pubsub/v2 to fix CI

[nothing0012]

Summary

Adds a notification webhook system to Flagr that fires on every flag CRUD operation. A single generic webhook provider sends a structured JSON payload to a configurable URL — simple, extensible, and dependency-light.

Changes

New: pkg/notification/

File	Purpose
notifier.go	Notifier interface, SendNotification(), MockNotifier, Notification struct
dispatch.go	Async dispatch with semaphore (max 100), panic recovery, statsd metrics
webhook.go	Generic HTTP POST webhook with custom headers, retry, io.LimitReader safety
retry.go	Exponential backoff with jitter for 5xx errors
validate.go	Startup config validation with warning logs

Modified

File	What
pkg/entity/flag_snapshot.go	SaveFlagSnapshot fires SendNotification after commit
pkg/handler/crud.go	Every CRUD op passes operation + component info (flag/segment/variant/constraint/distribution/tag)
pkg/config/env.go	7 new env vars (webhook URL, timeout, retry, diff toggle)
pkg/util/util.go	ParseHeaders() for webhook custom headers

Dependencies

• pmezard/go-difflib — for CalculateDiff() (unified diff of flag snapshots)

Webhook Payload

{
  "operation": "update",
  "flag_id": 123,
  "flag_key": "my-feature-flag",
  "component_type": "segment",
  "component_id": 7,
  "component_key": "power-users",
  "pre_value": "...",
  "post_value": "...",
  "diff": "--- Previous\n+++ Current\n@@ ...",
  "user": "admin@example.com",
  "timestamp": "2026-04-26T18:51:03Z"
}

pre_value, post_value, diff only appear when FLAGR_NOTIFICATION_DETAILED_DIFF_ENABLED=true

Configuration

Env var	Default	Description
FLAGR_NOTIFICATION_WEBHOOK_ENABLED	false	Enable webhook notifications
FLAGR_NOTIFICATION_WEBHOOK_URL	—	Target URL for POST
FLAGR_NOTIFICATION_WEBHOOK_HEADERS	—	Custom headers (Key: Val, Key: Val)
FLAGR_NOTIFICATION_TIMEOUT	10s	Per-notification context timeout
FLAGR_NOTIFICATION_DETAILED_DIFF_ENABLED	false	Include pre/post value and unified diff
FLAGR_NOTIFICATION_MAX_RETRIES	3	Retry attempts for 5xx errors
FLAGR_NOTIFICATION_RETRY_BASE	1s	Base delay for backoff with jitter
FLAGR_NOTIFICATION_RETRY_MAX	10s	Max delay between retries

Design Decisions

• Webhook-only — no Slack/Email providers. The webhook is the most general transport; users can point it at Slack incoming webhooks, SendGrid APIs, or any custom service. Adding providers later requires only implementing the Notifier interface.
• snake_case JSON with omitempty on optional fields
• flag_id/flag_key not entity_* — avoids confusion with Flagr's evaluation entity concept
• component_type/component_id/component_key — tracks which sub-component was modified
• No description — the flag's static description isn't useful context for a change event; operation + component + diff already tell the full story
• Privacy-by-default — diffs disabled unless explicitly opted in
• Async, non-blocking — notifications fire in background goroutines, failures never affect API responses
• Context timeout per notification, no redundant HTTP client timeout
• Jitter on exponential backoff to avoid thundering herds

Tests

27/27 passing across pkg/notification, pkg/handler, pkg/entity — covering dispatch, retry (with body leak tracking), webhook payload format, concurrency safety, and handler integration.

Types of changes

• New feature
• Documentation