Dev: Write up database core tables and schema - #6

ℹ️ Issue

Closes #3

📝 Description

Adds the BHO database schema. The doc describes each table, and the migration creates the tables in Postgres.

1. docs/database-schema.md lists every table and column, with the spreadsheet cell each column comes from.
2. db/migrations/0001_init_schema.ts is a Kysely migration. It creates users, stations, imports, daily_observations, hourly_observations, scheduled_observations, daily_records, and audit_log, and adds station 19737-02. Deleting a day also deletes its hourly and scheduled rows. down drops everything.
3. A trigger on the four weather tables writes one audit_log row per insert, update, or delete. To record who made a change, the writer calls set_config('app.user_id', ..., true) and set_config('app.import_id', ..., true) in its transaction.
4. db/types.ts has the Kysely type for each table.
5. db/client.ts has createDb(), which returns date columns as 'YYYY-MM-DD' strings. By default pg returns a Date at local midnight, so 2026-07-01 is 04:00Z on a laptop in Eastern time and 00:00Z in Lambda.
6. Adds kysely@^0.27.0 to the root package.json, the same version the Lambdas use.

Only audit_log.action is an enum. Values that may change, like imports.status, are text.

✔️ Verification

Ran the migration against Postgres 16 in Docker with Kysely's Migrator and FileMigrationProvider pointed at db/migrations. Until #11 adds yarn db:migrate, that is the way to run it. Checked that:

• up, down, and up again succeed, and down leaves no tables, type, or function behind.
• Deleting a day deletes its hourly and scheduled rows.
• Inserts, an update, and the cascaded deletes each wrote an audit_log row with the row's key and the user.
• With TZ=America/New_York, createDb() reads 2026-07-01 back as the string "2026-07-01".

🏕️ (Optional) Future Work / Notes

• hourly_observations.mountainVis (1@1.5) stays text. The client says it is mountain number @ visibility on a 1-3 scale. Once the weather group says which number is which mountain, split it into mountainId and mountainVisibility with a mountains table.
• On 4 days in July the peak gust has two times (28 WNW @ 1631E, 1651E), and July 19 has two directions. peakGustDir and peakGustTime hold one value each.
• daily_observations.remarks reads G47-G49, but notes also appear in G50-G54 on some days.
• importId is required on daily_observations and daily_records, which assumes only uploads write weather data. If staff can edit values in the app, it needs to be nullable.
• Re-uploading a month logs every row as an update, even when only importId changed.