Rewrite fasting trend algorithm and add comprehensive tests

- Replace day-iteration algorithm with pair-based approach for fasting
  detection. Iterate consecutive food scrobble pairs and mark foodless
  days between them as fasting days.

- Handle consecutive-day periodic fasting: when two food scrobbles are on
  consecutive days with a gap >= periodic_threshold but < full_threshold,
  mark the later day as a periodic fast (user's original bug fix).

- Fix _dt test helper to use replace() instead of subtracting hours,
  producing correct timestamps (e.g., '3 days ago at 18:46' instead of
  '3 days + 18h46m ago').

- Fix total_days off-by-one: removed +1 from inclusive day count so a
  'last_30' period correctly reports 30 days.

- Add _drinks_on_day() for per-day drink detection (replaces whole-
  window _drinks_in_window for more accurate liquid fasting detection).

- Fix Beer test fixture to use styles M2M field instead of non-existent
  beer_style attribute.

- Add 26 test cases covering: classification, short gaps, periodic/
  full/liquid fasting, multi-day gaps, vacation exemptions, custom
  thresholds, streaks, mixed types, edge cases.

Reasoning:
The original day-iteration algorithm had fundamental issues: it marked
days with food as fasting (since it only checked prev/next food times),
it didn't filter out normal (< periodic) gaps, and the _dt helper was
misinterpreting hour parameters. The new pair-based approach is cleaner:
for each consecutive food scrobble pair, we mark the foodless days
between them. This naturally handles multi-day fasts and correctly
avoids marking food-eating days as fasting. The consecutive-day periodic
rule preserves the user's original intent of detecting overnight fasts
between meals.

PROJECT.org

@@ -88,7 +88,7 @@ fetching and simple saving.
 *** Metadata sources
 **** Scraper
 
-* Backlog [0/23] :vrobbler:project:personal:
+* Backlog [1/24] :vrobbler:project:personal:
 ** TODO [#C] After transition to linux add curl_cffi as webpage scrapper again :webpages:metadata:
 ** TODO [#C] Create small utility to clean up tracks scrobbled with wonky playback times :bug:music:scrobbles:
 :PROPERTIES:
@@ -600,6 +600,11 @@ The Edit log form should have from top to bottom:
 
 *** Description
 
+** TODO [#A] Add trends tests for concurrent trends :trends:tests:concurrent:
+** DONE [#A] Add trends tests and fix fasting :trends:tests:fasting:
+:PROPERTIES:
+:ID:       70c20539-110e-d4f4-30bd-e245152459c2
+:END:
 * Version 61.2 [1/1]
 ** DONE Fix bug in trends and add liquid fasting :food:fasting:trends:
 :PROPERTIES: