docs(observability): document local Langfuse setup

Add the README section and .env.langfuse.example template for running
Langfuse locally via Compose. The Compose file itself is tracked
separately.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -0,0 +1,51 @@
+# Talie chatbot service
+
+Architecture decisions live in [`docs/adr`](docs/adr). The first implementation
+milestone is documented in the [ingestion vertical-slice plan](docs/plans/001-ingestion-vertical-slice.md).
+
+## Local Langfuse
+
+This repo includes a root-level development Compose file for Langfuse:
+
+- [`docker-compose.langfuse.yml`](docker-compose.langfuse.yml)
+- [`.env.langfuse.example`](.env.langfuse.example)
+
+Start Langfuse locally:
+
+```bash
+cp .env.langfuse.example .env.langfuse
+# edit .env.langfuse and replace CHANGE_ME values
+
+docker compose --env-file .env.langfuse -f docker-compose.langfuse.yml up -d
+```
+
+Open:
+
+```text
+http://localhost:3000
+```
+
+If the chatbot app runs on your host machine, configure it with:
+
+```env
+LANGFUSE_HOST=http://localhost:3000
+```
+
+If the chatbot app later runs inside the same Compose project/network as
+Langfuse, configure it with:
+
+```env
+LANGFUSE_HOST=http://langfuse-web:3000
+```
+
+A future app stack can be launched together with Langfuse using multiple Compose
+files:
+
+```bash
+docker compose \
+  -f docker-compose.yml \
+  -f docker-compose.langfuse.yml \
+  --env-file .env \
+  --env-file .env.langfuse \
+  up -d
+```