Setup tasks framework and delegate pmtiles export to a task on save

[TylerFisher]

Generates the public PMTiles export asynchronously instead of in the Django process.

• Django now triggers a lambda when CovenantedParcel changes.
• The Lambda (mp-covenants-pmtiles-export, separate repo) reads PostGIS directly, bakes the tiles with tippecanoe, uploads to S3, and POSTs back to a new token-authenticated callback endpoint (/api/pmtiles-callback/) that creates the PMTilesExport row.
• Two new settings: PMTILES_LAMBDA_NAME (blank disables the trigger, keeping tests/local dev offline) and PMTILES_CALLBACK_TOKEN.
• Schema parity between build_gdf and the Lambda's SQL is pinned by a shared fields.json + a test on both sides.
• Requires the Lambda to be deployed with VPC access to Postgres, an S3 gateway endpoint, and a reachable callback URL before the trigger does anything.