docs: clear review-round-3 nits — stale login_flow_v2, duplicates, field comments

Five small findings from the reviewer's third round, plus a SonarCloud
quality-gate failure on a test fixture.

- docs/troubleshooting.md, docs/configuration.md: six pre-PR references
  to a non-existent `login_flow_v2` mode value (the actual enum value is
  `login_flow`). They predated this PR but became actively misleading
  once `detect_auth_mode` started raising ValueError for anything not in
  the mode_map. Replaced with `login_flow` via sed.
- docs/configuration-migration-v2.md: removed a duplicate
  `MCP_DEPLOYMENT_MODE=multi_user_basic` line in the troubleshooting
  section (around line 447) — same shape as the round-2 duplicate
  caught earlier in the migration-steps section. Also dropped the
  `oauth_token_exchange` row from the mode-value table around line 364
  (that enum value was removed in 57303135 and would now raise
  ValueError from detect_auth_mode).
- nextcloud_mcp_server/config.py: field comments for
  `enable_multi_user_basic_auth` and `enable_login_flow` said
  "Auto-set by detect_auth_mode()" but the derivation moved into
  `Settings.__post_init__` in the previous commit. Updated both.
- tests/unit/test_config_validators.py: SonarCloud's python:S2068
  flagged `nextcloud_password="hunter2"` in the
  `test_login_flow_mode_auto_derives_enable_login_flow_flag` fixture I
  added in commit 5 as a potentially hard-coded credential. Other
  fixtures in the same file use the literal `"password"` and aren't
  flagged (they predate the PR and SonarCloud only checks new-code).
  Switched to `"password"` to match the existing convention.

No functional changes; all 1009 unit tests still pass.

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

docs/configuration-migration-v2.md

@@ -360,8 +360,7 @@ NEXTCLOUD_HOST=https://nextcloud.example.com
 |-----------|-------------|
 | `single_user_basic` | Single-user with username/password |
 | `multi_user_basic` | Multi-user with BasicAuth pass-through |
-| `login_flow` | Multi-user OAuth (recommended) |
-| `oauth_token_exchange` | Multi-user OAuth with token exchange |
+| `login_flow` | Multi-user OAuth via Login Flow v2 (recommended) |
 
 ### Mode Detection Priority
 
@@ -445,7 +444,6 @@ Server activates `login_flow` mode when you expected `multi_user_basic`
 Add explicit mode declaration:
 ```bash
 MCP_DEPLOYMENT_MODE=multi_user_basic
-MCP_DEPLOYMENT_MODE=multi_user_basic
 ```
 
 ---