feat(11-03): broadcast model writes through River jobs enqueued in the write transaction

- Broadcastable contract and Bind[T] bindings; event {action}.{alias},
  default {model, actor, timestamp, ttl} payload, delete snapshot taken
  before the row goes
- GORM callbacks installed via lagoon.OnDatabase enqueue a summer.broadcast
  job on the write's *sql.Tx inside a savepoint; failures are logged and
  never abort the write; zero-key batch writes are skipped
- WithoutBroadcasting[T] (ctx-scoped, per type) and Service.Emit for one
  summary event; the one-attempt job namespaces channels and publishes or
  broadcasts; the payload travels as a JSON string so JSONB keeps its order
- no jobs for the null driver or Centrifugo without an API key

modules/lighthouse/lighthouse.go

@@ -8,14 +8,18 @@
 package lighthouse
 
 import (
+	"database/sql"
 	"fmt"
 	"log/slog"
+	"reflect"
 	"strings"
 	"sync"
 	"time"
 
 	"git.golem15.com/golem15/summercms/modules/backpack"
 	"git.golem15.com/golem15/summercms/modules/compass"
+	"git.golem15.com/golem15/summercms/modules/lagoon"
+	"gorm.io/gorm"
 )
 
 const (
@@ -39,8 +43,9 @@ type Service struct {
 	queue     string
 	timeout   time.Duration
 
-	mu     sync.RWMutex
-	lookup UserLookup
+	mu       sync.RWMutex
+	lookup   UserLookup
+	bindings map[reflect.Type]*handler
 }
 
 // From returns the app's Service, building and publishing it on first use.
@@ -48,7 +53,10 @@ type Service struct {
 // realtime.broadcast_namespace (default ""), realtime.broadcast_queue
 // (default "broadcasts") and realtime.broadcast_timeout (default 5s; an
 // integer number of seconds or a duration string) and builds the driver
-// through its registered factory. An unknown driver name is an error.
+// through its registered factory. An unknown driver name is an error. It
+// also registers the broadcast job with the app's conga manager (so call it
+// from Boot, before a worker starts) and installs the broadcast GORM
+// callbacks through lagoon.OnDatabase.
 func From(app *backpack.App) (*Service, error) {
 	if app == nil {
 		return nil, fmt.Errorf("lighthouse: app is nil")
@@ -75,6 +83,16 @@ func From(app *backpack.App) (*Service, error) {
 		return nil, fmt.Errorf("lighthouse: driver %s returned nil", name)
 	}
 	svc.driver = d
+	if err := svc.registerJob(); err != nil {
+		return nil, fmt.Errorf("lighthouse: register broadcast job: %w", err)
+	}
+	// The broadcast callbacks need the GORM handle, which serve publishes
+	// after plugins boot.
+	if err := lagoon.OnDatabase(app, func(_ *sql.DB, gdb *gorm.DB) error {
+		return svc.installCallbacks(gdb)
+	}); err != nil {
+		return nil, fmt.Errorf("lighthouse: install broadcast callbacks: %w", err)
+	}
 	if err := app.Publish(svc); err != nil {
 		if existing, ok := app.Lookup[*Service](); ok && existing != nil {
 			return existing, nil