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.

docs/WORKLOAD_REFACTOR_TODO.md

@@ -25,10 +25,21 @@ order.
 > already had ≥87% coverage from the trigger-split work.
 >
 > **What's next** is open — the remaining items in the doc are nice-to-
-> haves (richer kind-aware UI forms for new trigger kinds; a /triggers
-> deep-link from the proxies page; more compose-source coverage that
-> needs a `compose` exec seam). Pick from the task list or close the
-> arc.
+> haves (a /triggers deep-link from the proxies page; more compose-source
+> coverage that needs a `compose` exec seam). Pick from the task list or
+> close the arc.
+>
+> **Trigger kind expansion (2026-05-16):** added the fourth trigger
+> kind, **schedule** — interval-based recurring trigger driven by the
+> new `internal/scheduler` tick loop (default 30s, ≤5m). v1 takes a
+> Go-duration interval ("24h", "1h", "168h") with a 1-minute floor;
+> dispatches through the same `webhook.Handler.FanOutForTrigger` seam
+> the inbound HTTP webhook uses, so per-binding concurrency / outcome
+> accounting / config-merge semantics are identical. `triggers` gained
+> a `last_fired_at` column; the scheduler persists it BEFORE dispatch
+> so a panicking Match cannot wedge a tight loop. The frontend
+> picker grid grew to four columns and `/triggers/[id]` surfaces
+> "last fired" on schedule rows.
 
 ## Status at a glance