feat(submissions): generated-doc filename <date> <keyword> (<case>) + user-replaceable keyword

Generated documents now download as "YYYY-MM-DD keyword (case number).docx"
(date first/sortable, case number bracketed) instead of the old
"rule-case-date.docx" shape.

- submissionFileName: date-led frame; keyword = user override > lang-aware
  rule name > "submission"; case number always bracketed, placeholder
  "Az. folgt" (named const) when the project has no Aktenzeichen.
- SanitiseSubmissionFileName hardened to fold the full Windows-reserved
  set (colon star question angle pipe) on top of slash/backslash, while
  preserving spaces + parentheses so the assembled frame stays
  human-facing yet filesystem-safe.
- User-replaceable keyword stored in the draft's composer_meta jsonb
  (filename_keyword, no migration). Editor gains a "Stichwort (Dateiname)"
  input that placeholders the auto rule name and persists via the draft
  PATCH path. One-click /generate has no draft row -> keeps auto keyword.

Tests: submissionFileName (full / no-AZ / override / EN / slash case-no /
blank override / empty rule), submissionFilenameKeyword, extended
sanitiser cases.

t-paliad-354

pkg/docforge/docx/dotm.go

@@ -185,7 +185,12 @@ func SanitiseSubmissionFileName(s string) string {
 	s = umlautFolder.Replace(s)
 	s = strings.Map(func(r rune) rune {
 		switch r {
-		case '/', '\\':
+		// Path separators and the rest of the Windows-reserved set —
+		// fold to underscore so a case number like "UPC_CFI_123/2026"
+		// stays one filesystem-safe segment. Spaces and parentheses are
+		// intentionally preserved: the human-facing download name
+		// "<date> <keyword> (<case>)" relies on them (t-paliad-354).
+		case '/', '\\', ':', '*', '?', '<', '>', '|':
 			return '_'
 		case '"', '\'':
 			return -1

pkg/docforge/docx/dotm_test.go

@@ -241,9 +241,12 @@ func TestSanitiseSubmissionFileName(t *testing.T) {
 		"Klageerwiderung":        "Klageerwiderung",
 		"Berufungsbegründung":    "Berufungsbegruendung",
 		"Schriftsatz/Anlage":     "Schriftsatz_Anlage",
-		`Statement of "Defence"`: "Statement of Defence",
-		`  Klage  `:              "Klage",
-		"Größe":                  "Groesse",
+		`Statement of "Defence"`:  "Statement of Defence",
+		`  Klage  `:               "Klage",
+		"Größe":                   "Groesse",
+		"UPC_CFI_123/2026":        "UPC_CFI_123_2026",
+		"a:b*c?d<e>f|g":           "a_b_c_d_e_f_g",
+		"Klageerwiderung (Frist)": "Klageerwiderung (Frist)",
 	}
 	for in, want := range cases {
 		t.Run(in, func(t *testing.T) {