Add the Web Push sender for the push channel (Feature 19)

push.Sender implements notify.Sender for the "push" channel with
webpush-go and the VAPID key pair: it sends {title, body, url, tag} to
every stored subscription with a TTL of 12 hours, one request each.
Per subscription, an accepted notification records last_success_at, any
failure increments failure_count, and a 404 or 410 deletes the
subscription. Send succeeds when one device accepted it or when there
are no subscriptions (logged as nothing to send); otherwise it returns
an error so the outbox retries. Errors and logs name subscriptions by
id; a transport *url.Error is unwrapped so the endpoint is never quoted.

A mailto: VAPID subject is passed to webpush-go as its bare address,
since webpush-go prefixes mailto: itself.

The server registers the sender with the notify service.

Maestro task 32a1a4ad-9221-46de-8468-3ffce19b108c.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/querier.go

@@ -68,6 +68,8 @@ type Querier interface {
 	DeleteAIRunsBefore(ctx context.Context, cutoff string) (int64, error)
 	DeleteIntegrationsGoogle(ctx context.Context) error
 	DeletePushSubscription(ctx context.Context, endpoint string) (int64, error)
+	// Removes a subscription the push service reports gone (404 or 410).
+	DeletePushSubscriptionByID(ctx context.Context, id string) error
 	DeleteRecipeIngredients(ctx context.Context, recipeID string) error
 	DeleteRecipeSteps(ctx context.Context, recipeID string) error
 	DeleteShoppingGroup(ctx context.Context, id string) error
@@ -188,6 +190,9 @@ type Querier interface {
 	// The planned dinners dated from from_date up to but not including
 	// to_date, oldest first. Days with no dinner are left out.
 	ListPlannedDinners(ctx context.Context, arg ListPlannedDinnersParams) ([]ListPlannedDinnersRow, error)
+	// Every stored subscription with its keys, for the push sender, oldest
+	// first.
+	ListPushSubscriptions(ctx context.Context) ([]ListPushSubscriptionsRow, error)
 	// Reads for the AI household context block (internal/ai/harness).
 	// The most recent names the household turned down, newest first, as
 	// ListPassedNames has them.
@@ -255,6 +260,10 @@ type Querier interface {
 	// event id while its dinner stays the same; a new dinner clears it, since
 	// Calendar no longer shows the day as planned.
 	PutPlanDay(ctx context.Context, arg PutPlanDayParams) error
+	// Counts a delivery to a subscription that the push service did not accept.
+	RecordPushFailure(ctx context.Context, id string) error
+	// Records that the push service accepted a notification for a subscription.
+	RecordPushSuccess(ctx context.Context, arg RecordPushSuccessParams) error
 	// Records a new draft over plan id: its status and note, raising its
 	// revision.
 	RedraftWeekPlan(ctx context.Context, arg RedraftWeekPlanParams) error