Deploy: main into prod (#343)

Brings `prod`, which upaas deploys, up to `main` at `9cf9cdd`, the merge of #321. `prod` was cut from `main` at `251cb3d` (1.0.0b1).

What it deploys is everything listed in #321. For running it:

- With `WEBHOOKER_ENVIRONMENT` unset, the instance runs as `prod` and sends no `Access-Control-Allow-Origin: *`.
- Each event database gains its new indexes the first time it is opened after the upgrade.
- `webhooker_delivery_retries_total` no longer counts a circuit breaker holding back a delivery that is already `retrying`.

Not in this PR yet: #340, in which the container sets its own data directory owner and mode before start. It is in progress on `next`. Once it reaches `main`, this PR carries it, because the PR follows `main`.

Model: opus-5-5
Co-authored-by: Jeffrey Paul <1+sneak@noreply.example.org>
Reviewed-on: #343

internal/delivery/target_headers_test.go

@@ -11,10 +11,11 @@ import (
 	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
-// Literals these tests repeat, named so that the header name and the
+// Literals these tests repeat, named so that the header names and the
 // keep-forever archive config each have one definition.
 const (
 	headerAuthorization = "Authorization"
+	headerContentType   = "Content-Type"
 	bearerValue         = "Bearer abc"
 	archiveConfigNever  = "{\"expiry\":\"never\"}"
 )