feat(api): phase 5 — alerts, agent depth, subdomains, auth, boards

P5b: notifier transport registry (webhook/slack/discord/telegram/
gotify/ntfy/smtp via net/smtp), per-monitor+domain alert rules
(failsBeforeDown, latencyWarn, certDays, mute).

P5c: per-container cpu/mem, optional SMART/ZFS/GPU exec-or-skip
collectors, crt.sh subdomain discovery with daily sweep.

P5d: opt-in auth (bcrypt, sqlite sessions, 7d rotating cookie, login
rate limit, public-route bypass) and multiple boards (boards table,
sections.board_id, /api/boards CRUD, slug routing).

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/db/migrations/0007_alerts.sql

@@ -0,0 +1,10 @@
+-- +goose Up
+-- Per-target alert rules as a JSON blob: {consecutiveFailures, latencyWarnMs,
+-- certDays, mute}. '{}' = defaults (alert on first failure, no latency rule,
+-- cert threshold = alert_days_before, unmuted).
+ALTER TABLE monitors ADD COLUMN alerts JSON NOT NULL DEFAULT '{}';
+ALTER TABLE domains  ADD COLUMN alerts JSON NOT NULL DEFAULT '{}';
+
+-- +goose Down
+ALTER TABLE monitors DROP COLUMN alerts;
+ALTER TABLE domains  DROP COLUMN alerts;