add: DB-backed settings UI for live-editable tunables

Add a settings page (/settings) and API (/api/settings GET/PUT) to edit
encoding profiles (crf/preset per media type), lp, OMDb key, log retention,
and delete-originals from the browser, persisted to a settings table in
logs.db. config.yaml seeds the store on first run; afterwards the DB is the
source of truth (paths.* and http_addr stay config-only). Each job snapshots
the current settings, so changes apply to the next encode with no restart.

PUT validates ranges (crf 0-63, preset 0-13, lp >=0, retention >=1) before
persisting. No auth, by design for now (isolated network) — see #2.

Closes #1

internal/settings/settings_test.go

@@ -0,0 +1,46 @@
+package settings
+
+import (
+	"testing"
+
+	"av1dae/pkg/types"
+)
+
+func valid() Settings {
+	return Settings{
+		DVD: Profile{30, 2}, Bluray: Profile{29, 3}, WebDL: Profile{30, 3}, TVRip: Profile{32, 2},
+		LP: 0, LogRetentionDays: 7,
+	}
+}
+
+func TestValidate(t *testing.T) {
+	if err := valid().Validate(); err != nil {
+		t.Fatalf("valid settings rejected: %v", err)
+	}
+	bad := func(mut func(*Settings)) Settings { s := valid(); mut(&s); return s }
+	cases := map[string]Settings{
+		"crf too high":    bad(func(s *Settings) { s.DVD.CRF = 64 }),
+		"crf negative":    bad(func(s *Settings) { s.Bluray.CRF = -1 }),
+		"preset too high": bad(func(s *Settings) { s.WebDL.Preset = 14 }),
+		"lp negative":     bad(func(s *Settings) { s.LP = -1 }),
+		"retention zero":  bad(func(s *Settings) { s.LogRetentionDays = 0 }),
+	}
+	for name, s := range cases {
+		if err := s.Validate(); err == nil {
+			t.Errorf("%s: expected validation error, got nil", name)
+		}
+	}
+}
+
+func TestProfileFor(t *testing.T) {
+	s := valid()
+	if s.ProfileFor(types.MediaTypeBluRay) != s.Bluray {
+		t.Error("bluray profile mismatch")
+	}
+	if s.ProfileFor(types.MediaTypeWebDL) != s.WebDL {
+		t.Error("webdl profile mismatch")
+	}
+	if s.ProfileFor(types.MediaType("anything-else")) != s.DVD {
+		t.Error("unknown media type should fall back to DVD")
+	}
+}