docs: address Login Flow v2 review feedback

Fix issues raised by reviewer on PR #743:

- troubleshooting.md: renumber "Getting Help" steps (4→3, 5→4) after
  earlier consolidation left a gap
- installation.md: drop stale "OIDC app" prerequisite; admin access is
  now optional under Login Flow v2 (works on stock Nextcloud 16+)
- semantic-search-architecture.md: rename VECTOR_SYNC_ENABLED to
  ENABLE_SEMANTIC_SEARCH in the Status callout (renamed in v0.58.0)
- configuration.md: remove Quick Start references to deprecated
  oauth-multi-user / oauth-advanced templates and point to
  login-flow-v2.md; update "OAuth, Multi-User BasicAuth" label to
  "Login Flow v2, Multi-User BasicAuth"
- auth-flows.md: fix background-sync diagram so Encrypt+persist step
  no longer crosses into the Nextcloud column

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/auth-flows.md

@@ -94,8 +94,8 @@ Astrolabe                     MCP Server                   Nextcloud
     │                             │                            │
     │── Store app password ──────▶│                            │
     │   (via management API)      │                            │
-    │                             │── Encrypt + persist ──────▶│
-    │                             │   (SQLite, Fernet)         │
+    │                             │ [Encrypt + persist locally]│
+    │                             │  (SQLite, Fernet)          │
     │◀── Confirmation ────────────│                            │
     │                             │                            │
     │         [Background job]    │                            │