Replace Historic View with animated CitiBike Trip Playback overlay

[Copilot]

Removes the Historic View mode entirely and replaces it with a Trip Playback mode that streams real CitiBike trip data as animated Google Maps overlays — always keeping 10 trips active simultaneously.

Data layer (frontend/trip_data.py — new)

• Dynamically builds month list from Jan 2024 → today; downloads s3.amazonaws.com/tripdata/YYYYMM-citibike-tripdata.csv.zip on demand
• Caches extracted trips locally as gzip CSVs in processed_data/trips/ (gitignored); skips already-cached months
• Samples up to 5 000 trips/month, filters to valid durations (1 min–3 hr), returns a started_at-sorted DataFrame

App changes (frontend/app.py)

• Removed: fetch_timestamps, fetch_artifact, add_historic_view_stations, and all Historic View sidebar/map logic
• Added: three cached helpers:
  • load_playback_trips() — wraps load_trips with Streamlit cache
  • get_trip_route(slat, slng, elat, elng) — single Google Maps Directions (bicycling) call, cached; coordinates rounded to 4 d.p. (~11 m) to maximise cache hits across trips sharing a station; falls back to straight line on API failure
  • compute_trip_routes(trips_df) — batch-computes routes for the preloaded slice
• Added: render_trip_playback() — injects a self-contained Google Maps JS animation via st.components.v1.html():
  • Starts 10 trips at t=0; simulation clock advances at speed × real time
  • Spotlight fade: ±15% window around the current dot is bright (opacity 0.9); far-past fades to 0.08; far-future shows faintly at 0.2
  • On trip end, replacement starts within a random [0, 10] simulated-second window to avoid burst restarts
  • HUD overlay shows active count, trip index, and current speed multiplier
• Sidebar: playback speed slider (1–200×), trips-to-preload slider (20–200), Start/Stop buttons

Original prompt

We need a whole new featureset. I want to completely remove the historic mode altogether. Instead, I want to use existing citibike's historic trip data to create some interesting overlays. https://s3.amazonaws.com/tripdata/index.html

First, download all citibike data from 2024 to latest, dynamically. Then, when users click playback, I want to select 10 trips, grab the google maps fastest path between them, and render them at a given playback speed. When a trip ends, select a random trip that starts (within a ~ 10 second interval of this one ending). This way, we'll always have 10 trips rendered. Trip rendering should use the google maps API to find a path between the stations specified, and then render the trip over time, fading out nodes that should have been passed and fading in new ones.

Created from VS Code.

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