feat: add Postgres store with append-only schema and migrations

internal/store connects via pgx and runs golang-migrate migrations
embedded in the binary (go:embed), so Deklarix stays a single binary
despite the move to Postgres. Schema covers the five MVP tables
(submission, asset, extraction, finding, evidence_package, participant).

extraction, finding and evidence_package are append-only by design: a
Postgres trigger rejects UPDATE/DELETE outright, since a corrigible
evidence archive isn't an evidence archive. Corrections to a finding are
new rows whose supersedes column points at the row they replace (set at
INSERT time on the new row, since the trigger blocks UPDATE on the old
one) — "currently valid" findings are the ones no other row supersedes.

scripts/test.sh now spins up a disposable Postgres container so the
store's integration tests (including the append-only guarantee) actually
run on every test.sh/release.sh invocation instead of silently skipping
for lack of DATABASE_URL.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

go.mod

@@ -1,3 +1,14 @@
 module github.com/netcell-it/deklarix
 
 go 1.26.6
+
+require (
+	github.com/golang-migrate/migrate/v4 v4.19.1 // indirect
+	github.com/jackc/pgerrcode v0.0.0-20220416144525-469b46aa5efa // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/pgx/v5 v5.10.0 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	golang.org/x/sync v0.18.0 // indirect
+	golang.org/x/text v0.31.0 // indirect
+)