feat(routes-f): recurring date series generator

[dmystical-coder]

Summary

Implements the recurring date series generator (#788).

POST /api/routes-f/recurring-dates with { start, frequency, interval?, count?, until? } → { dates, count }

Approach

• Supports daily | weekly | monthly | yearly frequencies with an optional positive interval (default 1).
• Each occurrence is computed directly from start (not incrementally), which avoids drift and keeps day-of-month anchoring correct.
• Month-end rollover: the day is clamped to the target month's last valid day but anchored to the original start day — Jan 31 monthly → Feb 29, Mar 31, Apr 30, May 31; Feb 29 yearly → Feb 28 on non-leap years and Feb 29 again on the next leap year.
• Hard cap of 1000 dates regardless of count/until; generation stops at whichever of count, until (inclusive), or the cap comes first. The loop is bounded by the cap so unbounded rules still terminate.
• Validates the frequency enum, positive-integer interval, valid ISO start/until, and until ≥ start.

Tests

Cover each frequency, interval handling, Jan-31 month-end rollover, Feb-29 yearly clamp/restore, until inclusivity, count/until/cap precedence, the unbounded cap, and the route's validation 400s.

Scope

All files live under app/api/routes-f/recurring-dates/; the only import is the existing in-folder _lib/validate helper, matching the merged fiscal-quarter/deep-merge routes.

Closes #788

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