feat: implement flight data scraping

[bhuvanh66]

Summary

Closes #7 Adds Flight data

1. CSO (Central Statistics Office, Ireland)
   Gives: monthly passenger numbers for Irish airports, by country, direction and flight type (table TAM07)
   Endpoint: https://ws.cso.ie/public/api.restful/PxStat.Data.Cube_API.ReadDataset/TAM07/CSV/1.0/en
   Auth: none

2. OpenSky Network
   Gives: every individual arrival at Dublin (EIDW). It has callsign, aircraft ID and estimated departure airport, but no airline, aircraft model or passenger info.
   Endpoint: https://opensky-network.org/api/flights/arrival
   Auth: OAuth2 client ID and secret (token from auth.opensky-network.org). The free tier gives 4,000 credits a day.

3. AeroDataBox (via RapidAPI)
   Gives: scheduled passenger arrivals into DUB, with airline, aircraft model, origin and status
   Endpoint: https://aerodatabox.p.rapidapi.com/flights/airports/iata/{airport}/{from}/{to}
   Auth: RapidAPI key. The free plan gives 400 units a month.

Added to example.env to put in api keys for these services

Type of change

• feat — new feature
• fix — bug fix
• docs — documentation only
• refactor — code change that neither fixes a bug nor adds a feature
• perf — performance improvement
• test — adding or updating tests
• chore / build / ci — tooling, deps, or pipeline changes

Changes

• Add CSO, Opensky, and AeroDataBox APIs

Database migrations

• No schema changes
• Includes an Alembic migration (just migrate-create "...")
• Migration runs cleanly (just migrate) and rolls back (just migrate-down)

Checklist

• Commits follow Conventional Commits
• Lint passes (just lint) and code is formatted (just format)
• Pre-commit hooks run clean (just hooks installed)
• Updated docs / README / .env.example where relevant
• Tested locally (just dev) and verified the API at http://localhost:8000/docs

How to test

Notes for reviewers