docs: address remaining Login Flow v2 review feedback

Round-2 cleanup of PR #743 review comments not covered by d153e96:

- configuration.md: fix broken `#multi-user-oauth-modes` anchor; replace
  with the two real anchors (Multi-User BasicAuth, Login Flow v2). Rewrite
  the stale "always use OAuth2/OIDC with pre-configured clients" Best
  Practices section to reflect the post-pivot mode matrix, and update the
  Docker volume example to mount the encrypted app-password store
  (`TOKEN_STORAGE_DB`) rather than obsolete `.oauth` client storage.
- semantic-search-architecture.md: rename remaining body references from
  the deprecated `VECTOR_SYNC_ENABLED` to `ENABLE_SEMANTIC_SEARCH` so the
  doc matches configuration.md / troubleshooting.md.
- running.md: relabel "OAuth Mode (Recommended)" as
  "Login Flow v2 / OAuth issuer mode (--oauth)", drop the misleading
  "(Legacy)" suffix from BasicAuth, drop the
  `NEXTCLOUD_OIDC_CLIENT_ID/SECRET` example (tied to the retired
  direct-OAuth-to-Nextcloud flow), and add a note explaining what
  `--oauth` actually enables post-pivot.
- keycloak-multi-client-validation.md, oauth-impersonation-findings.md:
  add a deprecation banner pointing at ADR-022 / Login Flow v2. Files
  retained because ADR-002 and CLAUDE.md still cite them.
- auth-flows.md: clarify under the Astrolabe → MCP diagram that the
  Nextcloud-OIDC JWKS path applies to Multi-User BasicAuth; under
  Login Flow v2 the MCP server validates tokens against its own JWKS.
- login-flow-v2.md: clarify the sticky-session note — affinity must key
  on the OAuth bearer token (or user-bound cookie), not source IP, since
  MCP clients may not maintain stable IPs across the provisioning flow.

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

docs/keycloak-multi-client-validation.md

@@ -1,5 +1,8 @@
 # Keycloak Multi-Client Token Validation
 
+> [!WARNING]
+> **Deprecated — historical reference only.** This document describes the direct-OAuth-to-Nextcloud architecture that was retired in favor of Login Flow v2. See [ADR-022](ADR-022-deployment-mode-consolidation.md) and [Login Flow v2](login-flow-v2.md) for the current approach. Retained because [ADR-002](ADR-002-vector-sync-authentication.md) and `CLAUDE.md` still cite this investigation for context.
+
 ## Executive Summary
 
 **Question**: Can Nextcloud's `user_oidc` app (configured with client A) validate bearer tokens from client B in the same Keycloak realm?