Make schedule/line-status logic timezone-aware (don't depend on server TZ)

[Jorioz]

Problem

Schedule-derived line status (#26/#27) computes the service date and service windows with a naive datetime.now(), and compares them against GTFS times that are agency-local (Eastern). This silently assumes the server's local timezone is the agency timezone.

It isn't in the container: the backend image runs in UTC, so the schedule was evaluated ~4–5 hours off and out-of-service lines never dimmed (or dimmed at the wrong time).

Diagnosis (from the debugging session)

At 05:12 UTC (= 01:12 Eastern), inside the container:

• provider with naive-UTC now → OUT: (none)
• same instant treated as Eastern → OUT: ['RH'] (correct)

The whole backend shares this naive-local-time assumption (live-feed timestamps, staleness, and now schedule status), so the app implicitly expects to run in Eastern.

Current mitigation (already shipped)

compose.yaml pins the backend container to TZ=America/Toronto. This is a deployment config fix, not a code fix — it works but breaks again if the service is ever deployed somewhere that doesn't set TZ, or run outside compose.

Proposed code-level fix

Make the schedule logic timezone-aware regardless of server TZ:

• Introduce an agency timezone constant (America/Toronto) via zoneinfo.
• Derive the service date and anchor service windows in agency-local time, then compare against an agency-local now (convert datetime.now(tz=...) or the injected clock into the agency zone).
• Keep the injected-clock seam so tests stay deterministic; add a test that pins a fake now in UTC and asserts the status matches agency-local expectations (i.e. the same instant that currently fails in UTC now resolves correctly without relying on TZ).

Acceptance criteria

• Line status is correct when the process runs in UTC (no TZ=America/Toronto set), covered by a test.
• Service date and window boundaries are computed in agency-local time (handles the >24:00 / past-midnight cases already covered by Dim a line after its last scheduled train of the day #27).
• The injected-clock seam and existing Dim lines with no scheduled service today #26/Dim a line after its last scheduled train of the day #27 tests still pass.
• Once the code is timezone-aware, the TZ=America/Toronto line in compose.yaml becomes belt-and-suspenders rather than load-bearing (note it, optionally keep it).

Blocked by

None. Independent of the shipped mitigation.