fix(backend): embed the IANA time-zone database

The taxpayer's zone decides which tax month an income is filed under, and the
config refuses to boot on a zone it cannot resolve rather than misfiling into
UTC. That correctness was resting on the base image happening to ship
/usr/share/zoneinfo — true of distroless/static-debian12 today, but not a thing
to leave implicit on a path where being wrong means a wrong tax return.

Costs about 450 KB of binary. Adds a config test pinning the default zone and
the hard failure on an unknown one.

backend/cmd/backend/main.go

@@ -15,6 +15,13 @@ import (
 	"syscall"
 	"time"
 
+	// The IANA time-zone database, embedded in the binary. The image ships one today, but the
+	// dependency is not one to leave implicit: the taxpayer's zone decides which tax month an
+	// income is filed under, and the config refuses to boot on a zone it cannot resolve rather
+	// than silently falling back to UTC and misfiling. Embedding it makes that correctness
+	// independent of whatever the base image happens to contain.
+	_ "time/tzdata"
+
 	"github.com/google/uuid"
 	"go.uber.org/zap"