feat: external importers + uptime monitors (merge plan M1)

Two tracks from the Beszel/Domain-Locker merge plan in ROADMAP.md:

- Importers: POST /api/import/external sniffs Homepage services.yml,
  Homarr JSON, and Dashy conf.yml into sections/items; SettingsDialog
  import now accepts all four formats.
- Monitors (M1): monitors + heartbeats schema, pure checkers for
  http/tcp/ping/dns/keyword/json-query/push, scheduler on app ctx,
  CRUD + check-now + push ingest API, /monitors + /monitors/:id pages
  (wouter), monitor board widget, ServiceDialog monitor opt-in.

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/0003_monitors.sql

@@ -0,0 +1,45 @@
+-- +goose Up
+-- Uptime monitors and their check history (merge phase M1).
+CREATE TABLE monitors (
+    id             TEXT PRIMARY KEY,
+    name           TEXT NOT NULL,
+    type           TEXT NOT NULL CHECK (type IN ('http','tcp','ping','dns','keyword','json','push')),
+    url            TEXT NOT NULL DEFAULT '',
+    hostname       TEXT NOT NULL DEFAULT '',
+    port           INTEGER NOT NULL DEFAULT 0,
+    method         TEXT NOT NULL DEFAULT 'GET',
+    headers        JSON NOT NULL DEFAULT '{}',
+    body           TEXT NOT NULL DEFAULT '',
+    keyword        TEXT NOT NULL DEFAULT '',
+    keyword_invert INTEGER NOT NULL DEFAULT 0,
+    json_query     TEXT NOT NULL DEFAULT '',
+    expected       TEXT NOT NULL DEFAULT '',
+    dns_type       TEXT NOT NULL DEFAULT 'A',
+    interval_s     INTEGER NOT NULL DEFAULT 60,
+    timeout_s      INTEGER NOT NULL DEFAULT 10,
+    retries        INTEGER NOT NULL DEFAULT 0,
+    active         INTEGER NOT NULL DEFAULT 1,
+    status         TEXT NOT NULL DEFAULT 'pending' CHECK (status IN ('up','down','pending','paused','maintenance')),
+    push_token     TEXT NOT NULL DEFAULT '',
+    tags           JSON NOT NULL DEFAULT '[]',
+    notes          TEXT NOT NULL DEFAULT '',
+    position       REAL NOT NULL DEFAULT 0,
+    last_check     TEXT,
+    created_at     TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+CREATE INDEX monitors_due ON monitors(active, last_check);
+
+CREATE TABLE heartbeats (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    monitor_id  TEXT NOT NULL REFERENCES monitors(id) ON DELETE CASCADE,
+    status      TEXT NOT NULL CHECK (status IN ('up','down')),
+    ping_ms     INTEGER NOT NULL DEFAULT 0,
+    msg         TEXT NOT NULL DEFAULT '',
+    cert_expiry INTEGER NOT NULL DEFAULT -1,
+    checked_at  TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+CREATE INDEX heartbeats_monitor_time ON heartbeats(monitor_id, checked_at);
+
+-- +goose Down
+DROP TABLE heartbeats;
+DROP TABLE monitors;