feat: Python supervisor + feature flags

Supervisor (replaces supervisor.sh):
- supervisor.py — cron-based pipeline orchestration, reads workflows.toml
  on every tick, runs due extractors in topological waves with parallel
  execution, then SQLMesh transform + serving export
- workflows.toml — workflow registry: overpass (monthly), eurostat (monthly),
  playtomic_tenants (weekly), playtomic_availability (daily),
  playtomic_recheck (hourly 6–23)
- padelnomics-supervisor.service — updated ExecStart to Python supervisor

Extraction enhancements:
- proxy.py — optional round-robin/sticky proxy rotation via PROXY_URLS env
- playtomic_availability.py — parallel fetch (EXTRACT_WORKERS), recheck mode
  (main_recheck) re-queries imminent slots for accurate occupancy measurement
- _shared.py — realistic browser User-Agent on all extractor sessions
- stg_playtomic_availability.sql — reads morning + recheck snapshots, tags each
- fct_daily_availability.sql — prefers recheck over morning for same slot

Feature flags (replaces WAITLIST_MODE env var):
- migration 0019 — feature_flags table, 5 initial flags:
  markets (on), payments/planner_export/supplier_signup/lead_unlock (off)
- core.py — is_flag_enabled() + feature_gate() decorator
- routes — payments, markets, planner_export, supplier_signup, lead_unlock gated
- admin flags UI — /admin/flags toggle page + nav link
- app.py — flag() injected as Jinja2 global

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

infra/supervisor/padelnomics-supervisor.service

@@ -7,10 +7,11 @@ Wants=network-online.target
 Type=simple
 User=root
 WorkingDirectory=/opt/padelnomics
-ExecStart=/opt/padelnomics/infra/supervisor/supervisor.sh
+ExecStart=/bin/sh -c 'exec uv run python src/padelnomics/supervisor.py'
 Restart=always
 RestartSec=10
 EnvironmentFile=/opt/padelnomics/.env
+Environment=PATH=/root/.local/bin:/usr/local/bin:/usr/bin:/bin
 Environment=LANDING_DIR=/data/padelnomics/landing
 Environment=DUCKDB_PATH=/data/padelnomics/lakehouse.duckdb
 Environment=SERVING_DUCKDB_PATH=/data/padelnomics/analytics.duckdb