Document spec gaps: auth, phase scaling, observability, testing

Address 21 previously undefined behaviors across specs:

- Authentication: Replace email/password with OIDC (Pocket-ID)
- Cycle tracking: Add fixed-luteal phase scaling formula with examples
- Calendar: Document period logging behavior (preserve predictions)
- Garmin: Clarify connection is required (no phase-only mode)
- Dashboard: Add UI states, dark mode, onboarding, accessibility
- Notifications: Document timezone batching approach
- New specs: observability.md (health, metrics, logging)
- New specs: testing.md (unit + integration strategy)
- Main spec: Add backup/recovery, known limitations, API updates

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

specs/calendar.md

@@ -55,6 +55,31 @@ Generates new `calendarToken`, invalidating previous subscriptions.
 }
 ```
 
+### Period Logging Behavior
+
+When a user logs a new period start date, the ICS feed updates as follows:
+
+1. **Current cycle**: Phase events are corrected retroactively to reflect actual dates
+2. **Original predictions preserved**: Previous predictions for the current cycle are kept as separate events with "(Predicted)" suffix, allowing users to compare predicted vs. actual
+3. **Older cycles**: Events from previous cycles remain unchanged
+4. **Future cycles**: Regenerated based on new period date and cycle length
+
+**Example after logging period on Day 28 of a predicted 31-day cycle:**
+```ics
+BEGIN:VEVENT
+SUMMARY:🩸 Menstrual (Days 1-3)
+DTSTART;VALUE=DATE:20240128
+DESCRIPTION:Actual period start
+END:VEVENT
+BEGIN:VEVENT
+SUMMARY:🩸 Menstrual (Predicted)
+DTSTART;VALUE=DATE:20240131
+DESCRIPTION:Original prediction - period arrived 3 days early
+END:VEVENT
+```
+
+This creates a built-in accuracy feedback loop for understanding cycle patterns.
+
 ## In-App Calendar
 
 ### Month View (`/calendar`)
@@ -107,6 +132,8 @@ Below calendar:
 - [ ] ICS contains events for next 90 days
 - [ ] Invalid token returns 401
 - [ ] Regenerate token creates new unique token
+- [ ] Period log updates current cycle events retroactively
+- [ ] Period log preserves original predictions as "(Predicted)" events
 - [ ] Month view renders all days in month
 - [ ] Day cells show correct phase colors
 - [ ] Today is visually highlighted