dateparser: python shell-out with two-step parse + russian qualifier pre-processing

PythonDateParser shells out to python3 with the dateparser library for
full natural-language date/time extraction. Two-step approach:
1. search_dates() finds the date substring in surrounding text
2. parse() re-parses the substring for correct time resolution

Russian time qualifiers (утра/вечера/дня/ночи) are pre-processed to
AM/PM before parsing — dateparser drops them during substring extraction.

Falls back to StubDateTimeParser when python3 or dateparser isn't
available (graceful degradation, no hard runtime dependency).

Dockerfile updated: python3 + dateparser==1.4.1 in runtime stage.

cmd/mavend/voice.go

@@ -229,7 +229,7 @@ func wireVoice(cfg *config.Config, coreAPI ipc.CoreAPI, phr phraser.Phraser, mem
 		memStore:         memStore,
 		dialogueSessions: dialogueSessions,
 		queryMinScore:    cfg.Voice.QueryMinScore,
-		timeParser:       router.StubDateTimeParser{},
+		timeParser:       router.NewPythonDateParser(),
 	}
 
 	// ----- the server (TCP listener) -----
@@ -803,7 +803,7 @@ func buildRouter(emb router.Embedder, acts router.ActMatcher, threshold float64)
 		Grammars:   grammars,
 		Classifier: cls,
 		Extractor: router.Extractor{
-			Time:  router.StubDateTimeParser{},
+			Time:  router.NewPythonDateParser(),
 			Acts:  acts,
 			Facts: router.DefaultFactParser{},
 		},