docs: reorganize research docs + add project tracker and marketing strategy

Move historical docs from docs/ and .claude/ to research/. Add superseded
notice to research/PLAN.md. Add CHANGELOG entries for previous fixes.
New: PROJECT.md (task tracker), docs/MARKETING.md (marketing strategy).

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

CHANGELOG.md

@@ -28,6 +28,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 - Template sync: copier update v0.9.0 → v0.10.0 — `export_serving.py` module,
   `@padelnomics_glob()` macro, `setup_server.sh`, supervisor export_serving step
 
+### Fixed
+- **Eurostat JSON-stat parsing** — API returns 4-7 dimension sparse dictionaries (583K values)
+  that caused DuckDB OOM; extractor now pre-processes JSON-stat into flat records with
+  configurable dimension filters per dataset
+- **Playtomic venue lat/lon** — staging model used wrong JSON path (`address.coordinate_lat`
+  vs actual `address.coordinate.lat`)
+- **dim_cities CTE** — unused `eurostat_labels` CTE caused `city_slug_raw` column not found error
+
 ### Removed
 - `extract/.../execute.py` — replaced by per-source modules
 - `models/raw/` directory — raw layer eliminated; staging reads landing files directly