feat: add institutional membership tiers and correct member directory

Corrections per 2026-04 institutional review:
- MLFO reclassified as Global Family Office (was incorrectly labeled central bank)
- BIS Innovation Hub reclassified as Standards Body (does not hold observer seat)
- Added missing entities: ICCC, SAID, PANDA, Order of Hospitallers (XOM)
- Added BRICS founding + expanded member central banks (10 entries)

New institutional tier taxonomy (7 tiers):
  sovereign_central_bank, global_family_office, settlement_member,
  infrastructure_operator, oversight_judicial, delegated_authority,
  standards_body

Backend changes:
- New auth/membership.go: tier types, DefaultTrackForTier mapping,
  MembershipStore with DB queries for member directory
- New migration 0017: institutional_members + institutional_member_wallets
  tables with seed data for all corrected members
- Updated wallet_auth.go getUserTrack(): now resolves institutional
  membership (via wallet junction table) before defaulting to Track 1
- WalletAuthResponse now includes institutional_tier and institution_name
- New REST endpoints: GET /api/v1/membership/{tiers,members,members/:slug}
- Added TrackLabel() helper in featureflags

Frontend changes:
- Added InstitutionalTier type and label map to access.ts
- WalletAccessSession extended with institutionalTier/institutionName
- Navbar getAccessTier() now displays institutional tier label when present
- Session summary shows institution name

Co-Authored-By: Nakamoto, S <defi@defi-oracle.io>

backend/database/migrations/0017_institutional_membership.down.sql

@@ -0,0 +1,5 @@
+-- 0017_institutional_membership.down.sql
+DROP TRIGGER IF EXISTS update_institutional_members_updated_at ON institutional_members;
+DROP TABLE IF EXISTS institutional_member_wallets;
+DROP TABLE IF EXISTS institutional_members;
+DROP TYPE IF EXISTS institutional_tier;