feat(auth,security,scheduling): JWT revocation, S3 fix, notes validation, docs, tests

- JWT revocation with JTI (UUID v4): in-memory tracking, POST /api/logout,
  refresh handler revokes old JTI, RequireAuth rejects revoked tokens
- Fix extractKey for S3 portfolio deletion: extracts full key path from URLs
  instead of just filename, preventing orphaned storage files
- Notes validation: max=1000000 on all 13 Notes fields across 4 booking structs
- CharCounter: grapheme-aware counter (Intl.Segmenter), threshold 750K,
  color-coded, integrated into 6 booking/admin components
- loginInProgress: timestamp-based tracking, 30s staleness, 20-entry cap (429),
  ticker cleanup for stuck entries
- Profile picture 15MB client-side limit, portfolio 20MB backend limit
- Exceptional scheduling: expand query start to Monday of week
- TodayCalendar: week-range fetching, closing time indicator, short-day lunch skip
- NavBar: link reorder, mobile burger badge, slide transition, backdrop
- ImageUpload: 20MB limit with visual feedback
- formatDateISO: shared YYYY-MM-DD utility, shouldApplyLunchProtection helper
- Update README.md and all Obsidian docs (Overview, Technical, Admin, Future Work)
- Add 28 new tests: JWT (11), auth handlers (7), portfolio extractKey (5),
  notes validation (5). go build + go vet clean with test,dev tags

backend/handlers/user/profile.go

@@ -75,7 +75,7 @@ type AdminUserDetail struct {
 	LastLoginAt      *string `json:"lastLoginAt,omitempty"`
 	CreatedAt        string  `json:"createdAt"`
 	UpdatedAt        string  `json:"updatedAt"`
-	Notes            *string `json:"notes,omitempty"`
+	Notes            *string `json:"notes,omitempty" validate:"omitempty,max=1000000"`
 
 	// GDPR consent fields
 	PrivacyPolicyConsent   bool    `json:"privacyPolicyConsent"`