feat(triggers): add schedule trigger kind + internal scheduler

Fourth trigger kind alongside registry/git/manual. Recurring time-interval
fires driven by a new internal/scheduler tick loop (default 30s, clamped
to 5m). Goes through the same webhook.Handler.FanOutForTrigger seam as
inbound HTTP webhooks, so per-binding concurrency, outcome accounting,
and config-merge semantics are identical.

Schema: triggers.last_fired_at TEXT column (additive ALTER for existing
DBs). Scheduler persists last_fired_at BEFORE dispatch so a panicking
Match cannot wedge a tight loop; failed deploys wait one full interval
before retry — correct trade-off for a periodic refresh trigger.

Frontend: TriggerKindForm + /triggers/new + /triggers/[id] gain the
schedule kind (4-col card grid, preset chips Hourly/Daily/Weekly,
custom interval input matched to Go time.ParseDuration syntax, optional
pinned reference). /triggers/[id] surfaces "last fired" on schedule rows.
EN+RU i18n in parity.

Review fixes from go-reviewer / security-reviewer / typescript-reviewer:
- Scheduler Start/Stop wrapped in sync.Once (no goroutine leak / double-
  cancel panic on shutdown re-entry).
- shouldFire rejects sub-MinInterval as defense-in-depth against
  hand-inserted rows that bypassed Validate.
- fire() asserts trigger Kind=="schedule" before dispatching.
- Aligned isValidInterval regex across all three frontend sites; reject
  the unsupported "d" unit (Go time.ParseDuration doesn't accept it).
- formatLastFired falls back to lastFiredNever on malformed timestamps
  rather than leaking raw bytes into the UI.
- main.go scheduler closure logs per-fire deployed/errored counts.

internal/store/models.go

@@ -366,8 +366,13 @@ type Trigger struct {
 	WebhookSecret           string `json:"-"`              // URL-identifier secret; never serialized
 	WebhookSigningSecret    string `json:"-"`              // HMAC key; never serialized
 	WebhookRequireSignature bool   `json:"webhook_require_signature"`
-	CreatedAt               string `json:"created_at"`
-	UpdatedAt               string `json:"updated_at"`
+	// LastFiredAt is the RFC3339 wall-clock the scheduler last dispatched
+	// this trigger. Empty for never-fired or non-schedule triggers. The
+	// scheduler reads + writes this column to decide next-fire windows
+	// and to surface "last fired" on the trigger detail page.
+	LastFiredAt string `json:"last_fired_at,omitempty"`
+	CreatedAt   string `json:"created_at"`
+	UpdatedAt   string `json:"updated_at"`
 }
 
 // WorkloadTriggerBinding joins a Workload to a Trigger. BindingConfig is