feat: DuckDB two-file architecture — resolve SQLMesh/web-app lock contention

Split the single lakehouse.duckdb into two files to eliminate the exclusive
write-lock conflict between SQLMesh (pipeline) and the Quart web app (reader):

  lakehouse.duckdb  — SQLMesh exclusive (all pipeline layers)
  serving.duckdb    — web app reads (serving tables only, atomically swapped)

Changes:

web/src/beanflows/analytics.py
- Replace persistent global _conn with per-thread connections (threading.local)
- Add _get_conn(): opens read_only=True on first call per thread, reopens
  automatically on inode change (~1μs os.stat) to pick up atomic file swaps
- Switch env var from DUCKDB_PATH → SERVING_DUCKDB_PATH
- Add module docstring documenting architecture + DuckLake migration path

web/src/beanflows/app.py
- Startup check: use SERVING_DUCKDB_PATH
- Health check: use _db_path instead of _conn

src/materia/export_serving.py (new)
- Reads all serving.* tables from lakehouse.duckdb (read_only)
- Writes to serving_new.duckdb, then os.rename → serving.duckdb (atomic)
- ~50 lines; runs after each SQLMesh transform

src/materia/pipelines.py
- Add export_serving pipeline entry (uv run python -c ...)

infra/supervisor/supervisor.sh
- Add SERVING_DUCKDB_PATH env var comment
- Add export step: uv run materia pipeline run export_serving

infra/supervisor/materia-supervisor.service
- Add Environment=SERVING_DUCKDB_PATH=/data/materia/serving.duckdb

infra/bootstrap_supervisor.sh
- Add SERVING_DUCKDB_PATH to .env template

web/.env.example + web/docker-compose.yml
- Document both env vars; switch web service to SERVING_DUCKDB_PATH

web/src/beanflows/dashboard/templates/settings.html
- Minor settings page fix from prior session

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/bootstrap_supervisor.sh

@@ -93,6 +93,7 @@ PULUMI_ACCESS_TOKEN=${PULUMI_ACCESS_TOKEN}
 PATH=/root/.cargo/bin:/root/.pulumi/bin:/usr/local/bin:/usr/bin:/bin
 LANDING_DIR=/data/materia/landing
 DUCKDB_PATH=/data/materia/lakehouse.duckdb
+SERVING_DUCKDB_PATH=/data/materia/serving.duckdb
 EOF
 
 echo "--- Setting up systemd service ---"