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/installation.md

@@ -6,7 +6,7 @@ This guide covers installing the Nextcloud MCP server on your system.
 
 - **Python 3.11+** - Check with `python3 --version`
 - **Access to a Nextcloud instance** - Self-hosted or cloud-hosted
-- **Administrator access** (for OAuth setup) - Required to install OIDC app
+- **Administrator access** *(optional)* - Only needed to customise app-password policies in Nextcloud settings; not required for any deployment mode (single-user, multi-user BasicAuth, or Login Flow v2)
 
 ## Installation Methods