Phase E: Data licensing, attribution, and privacy updates

[jlucus]

Part of milestone v2.1.0 — Community Data Pipeline. Full plan: docs/02-project-scope/DATA_PIPELINE_PLAN.md.

Acceptance criteria

• LICENSE-DATA documenting ODbL obligations for the OSM-derived database (attribution + share-alike)
• OSM attribution in the site footer
• Privacy policy section covering the optional submission email (processor, retention, deletion)
• README data-sources section with per-source licenses and refresh cadence