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.

web/src/lib/i18n/en.json

@@ -1083,7 +1083,9 @@
       "rotateConfirm": "Rotate now",
       "unbindTitle": "Unbind workload?",
       "unbindMessage": "Workload \"{name}\" will stop redeploying when this trigger fires. The workload itself is not deleted.",
-      "unbindConfirm": "Unbind"
+      "unbindConfirm": "Unbind",
+      "lastFired": "Last fired",
+      "lastFiredNever": "Never fired"
     },
     "form": {
       "kindLabel": "Kind",
@@ -1108,6 +1110,18 @@
       "branchPlaceholder": "main",
       "branchHint": "Only push events advancing this branch fire the trigger.",
       "manualNote": "Manual triggers carry no config. They fire only via the workload's Deploy button or POST /workloads/{id}/deploy.",
+      "scheduleNote": "Fires on a fixed interval driven by Tinyforge's internal scheduler. No external webhook is required — enable the webhook ingress below only if a CI also needs to fire it on demand.",
+      "intervalPresets": "Quick presets",
+      "intervalPreset": {
+        "hourly": "Hourly",
+        "daily": "Daily",
+        "weekly": "Weekly"
+      },
+      "interval": "Interval",
+      "intervalHint": "Go duration (e.g. \"30m\", \"6h\", \"24h\", \"168h\"). Minimum 1 minute.",
+      "scheduleReference": "Pinned reference (optional)",
+      "scheduleReferencePlaceholder": "stable",
+      "scheduleReferenceHint": "Optional tag, branch, or revision the source plugin should re-pull each fire. Leave empty to let the source use its default.",
       "unknownNote": "This kind has no built-in form yet. Use the JSON editor below; the server validates the shape.",
       "advancedToggle": "Advanced JSON",
       "advancedHint": "Power-user fallback — replaces the structured form with the raw config payload.",