feat: add Elasticsearch/Grafana date-math syntax to time filter

[pike00]

Summary

Adds Elasticsearch/Grafana-style date-math expressions (now-1y, now/M, now-1y/y, etc.) to the Fava time filter, with an explicit snap concept for period boundaries.

New syntax

now[+/-Nunit][/snap]

• Units: d (days), w (weeks), M (months), y (years)
• Snaps: /d, /w, /M, /y, /fy (fiscal year), /fQ (fiscal quarter)
• Ranges: now-30d - now, now/M - now, now-1y/y - now/y

Without an explicit snap, the unit determines the period (e.g. now-1M = last calendar month, equivalent to now-1M/M).

Changes

Backend (src/fava/util/date.py)

• DATEMATH_RE — regex for now[+/-N][unit][/snap]
• _snap_date() — snaps to d/w/M/y/fy/fQ boundaries
• _period_end_from_snap() — computes exclusive end from a snapped start
• _eval_datemath() — evaluates a single date-math expression
• _parse_datemath() — handles single and range date-math, tried before legacy parser
• parse_date_resolved() — returns (begin, end, "YYYY-MM-DD to YYYY-MM-DD") for UI/API use
• All legacy syntax (2024, year-1, FY2024, 2024 - 2025) preserved

Tests (tests/test_util_date.py)

• 24 new parametrized tests: date-math singles, fiscal snaps, ranges, resolved descriptions
• All 169 tests pass

Docs (docs/time-filter-syntax.md)

• Full date-math section with snap explanation and step-by-step walkthroughs

Backward compatibility

All existing syntax continues to work unchanged. Date-math is tried first, then falls through to the legacy parser.

Snap concept

A snap rounds a date down to the start of a period. Without a snap, now-1y means "exactly 1 year ago" (a single point). With a snap, now-1y/y means "the start of the year that was 1 year ago" (the full calendar year). Without an explicit snap, the unit implies the snap — now-1M is equivalent to now-1M/M.