C3: prune derived_prices rows older than the hot window (dry-run by default)

Daily retention prune: removes derived_prices rows older than derived_prices_hot_window_days (default 90d) across all known symbols.

DRY-RUN BY DEFAULT (execute=False) until the founder explicitly enables automated pruning by flipping the lambda to execute=True. Flipping to execute=True is a FOUNDER DECISION and is gated by C3 cutover preconditions (core_ohlc_serving_enabled=True + R2 export verified) enforced inside prune_derived.run — so even a mistaken flip raises before deleting if the cutover isn't complete. Deleted rows are irreversible.

pg_cron schedule: daily at 03:50 UTC (0004_schedule_prune_derived.sql). Dead-man 2x = 48h.