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/database/database.go

@@ -17,12 +17,12 @@ import (
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/banner"
 	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/datadir"
 	"sneak.berlin/go/webhooker/internal/gormlog"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
 const (
-	dataDirPerm       = 0750
 	randomPasswordLen = 16
 	sessionKeyLen     = 32
 )
@@ -185,7 +185,9 @@ func (d *Database) connect() error {
 // caller's decision.
 func (d *Database) connectTo(dataDir string) error {
 	// Ensure the data directory exists before opening the database.
-	err := os.MkdirAll(dataDir, dataDirPerm)
+	// datadir.DirPerm is the single source of the directory mode; this
+	// package creates the directory too, since either may run first.
+	err := os.MkdirAll(dataDir, datadir.DirPerm)
 	if err != nil {
 		return fmt.Errorf(
 			"creating data directory %s: %w",