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.

Dockerfile

@@ -51,7 +51,10 @@ FROM debian:trixie-slim AS runtime
 # zone and time.Now() stays UTC, and mavend answers clock/date queries and
 # evaluates quiet-hours in UTC.
 RUN apt-get update && apt-get install -y --no-install-recommends \
-      ca-certificates libvulkan1 mesa-vulkan-drivers libgomp1 tzdata && \
+      ca-certificates libvulkan1 mesa-vulkan-drivers libgomp1 tzdata \
+      python3 python3-pip && \
+    pip3 install --no-cache-dir --break-system-packages 'dateparser==1.4.1' && \
+    apt-get purge -y --auto-remove python3-pip && \
     rm -rf /var/lib/apt/lists/*
 
 # runtime native libs: whisper/ggml (incl. vulkan) are real files in deps/lib.