Add incremental TBA match cache with per-event If-Modified-Since support

[Copilot]

Every call to fetch_all_matches was doing a full re-fetch of all events and all matches, ignoring HTTP conditional-request semantics at the per-event level. Three root causes:

• A single if_modified_since timestamp (from the events-list response) was reused for every individual event match/team fetch — too coarse-grained.
• The result dict was replaced wholesale on each fetch, silently discarding previously cached matches and event_teams (noted in a TODO).
• A single try/except wrapped the entire event loop, so any ApiException(304) from one event aborted all remaining fetches.

Changes (current/backend/TBA.py)

• Per-event Last-Modified tracking — new event_last_modified dict in the cache stores the Last-Modified response header keyed by event.key after each successful fetch.
• Per-event conditional requests — each get_event_matches / get_event_teams call uses its own stored timestamp; TBA can return 304 for unchanged events independently.
• Per-event error isolation — each event fetch is wrapped in its own try/except; a 304 for one event keeps cached data and continues the loop.
• Fix cache merge — replaced the wholesale dict replacement with result.setdefault(...) so existing matches, event_teams, and event_last_modified are always preserved across fetches.
• Events-list 304 — handled gracefully by falling back to the cached events list rather than propagating an exception.
• reset=True — passes empty if_modified_since for every call to force a full re-fetch when needed.

# Before: same stale timestamp for every event
matches = api_instance.get_event_matches(e.key, if_modified_since=if_modified_since)

# After: per-event timestamp; 304 keeps cached data, loop continues
try:
    matches = api_instance.get_event_matches(event.key, if_modified_since=event_if_modified)
    result['matches'][event.key] = matches
    result['event_last_modified'][event.key] = api_instance.api_client.last_response.getheader('Last-Modified', '')
except ApiException as exc:
    if exc.status == 304:
        pass  # cached data retained

Structure

New current/ folder mirrors 2024/backend/ and serves as the active season backend. swagger_client is symlinked from 2024/backend/swagger_client to avoid duplication.

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