✨(agents) initialize LiveKit agent from multi-user transcriber example

Create Python script based on LiveKit's multi-user transcriber example
with enhanced request_fnc handler that ensures job uniqueness by room.

A transcriber sends segments to every participant present in a room and
transcribes every participant's audio. We don't need several
transcribers in the same room. Made the worker hidden - by default it
uses auto dispatch and is visible as any other participant, but having
a transcriber participant would be weird since no other videoconference
tool treats this feature as a bot participant joining a call.

Job uniqueness is ensured using agent identity by forging a
deterministic identity for each transcriber by room. This makes sure
two transcribers would never be able to join the same room. It might be
a bit harsh, but our API calling to list participants before accepting
a new transcription job should already filter out situations where an
agent is triggered twice.

We chose explicit worker orchestration over auto-dispatch because we
want to keep control of this feature which will be challenging to
scale. LiveKit agent scaling is documented but we need to experiment in
real life situations with their Worker/Job mechanism.

Currently uses Deepgram since Arnaud's draft Kyutai plugin isn't ready
for production. This allows our ops team to advance on deploying and
monitoring agents. Deepgram was a random choice offering 200 hours
free, though it only works for English. ASR provider needs to be
refactored as a pluggable system selectable through environment
variables or settings.

Agent dispatch will be triggered via a new REST API endpoint to our
backend. This is quite a first naive version of a minimal dockerized
LiveKit agent to start playing with the framework.

src/agents/Dockerfile

@@ -0,0 +1,24 @@
+FROM python:3.13-slim AS base
+
+FROM base AS builder
+
+WORKDIR /builder
+
+COPY pyproject.toml .
+
+RUN mkdir /install && \
+  pip install --prefix=/install .
+
+FROM base AS production
+
+WORKDIR /app
+
+ARG DOCKER_USER
+USER ${DOCKER_USER}
+
+# Un-privileged user running the application
+COPY --from=builder /install /usr/local
+
+COPY . .
+
+CMD ["python", "multi-user-transcriber.py", "start"]