Refactor documentation and diagrams for web/pipeline. (#272)

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com> for assistance with diagram creation and documentation prose updates.

docs/pipeline/diagrams/planned/activity.md

@@ -0,0 +1,16 @@
+# The Biergarten Data Pipeline — Activity Diagram (Planned)
+
+Each generation phase runs a producer/consumer chain concurrently: an
+**Orchestrator** producer feeds a bounded channel, an **LLM Worker** consumes it
+and generates records (closing its output channel when the input is drained and
+closed), and a **SQLite Worker** consumes that output channel and commits in
+batches. The brewery phase inserts a fourth stage, a **Geocode Worker**, between
+the LLM Worker and the SQLite Worker: reverse-geocoding is network-bound and
+rate-limited independently of LLM inference (Nominatim's 1-request-per-second
+policy), so keeping it off the LLM Worker's thread means neither stage stalls
+waiting on the other's bottleneck. The Checkin and Follow phases run in parallel
+with each other once the user and brewery pools are populated.
+
+```mermaid
+--8<-- "pipeline/diagrams/planned/activity.mmd"
+```