refactor(config)!: drop ENABLE_MULTI_USER_BASIC_AUTH env var, fail loud on legacy aliases

Same pattern as the ENABLE_LOGIN_FLOW removal in the previous commit:
the deployment mode (MCP_DEPLOYMENT_MODE) is the single source of truth
for selecting an auth flow. The ENABLE_MULTI_USER_BASIC_AUTH env-var
alias is redundant with `MCP_DEPLOYMENT_MODE=multi_user_basic`.

Unlike the ENABLE_LOGIN_FLOW removal — where silent removal was safe
because Login Flow v2 is the auto-detection default — silent removal
here would be a surprise: a user with only ENABLE_MULTI_USER_BASIC_AUTH=true
in their .env would auto-detect into LOGIN_FLOW after upgrade (wrong
runtime mode). Mitigation: detect_auth_mode now reads os.environ
directly for both legacy aliases and raises ValueError with a one-line
migration message if either is set. Applied retroactively to
ENABLE_LOGIN_FLOW as well — loud is better than silent.

- nextcloud_mcp_server/config.py:
  - Drop the dynaconf env-var alias entry for ENABLE_MULTI_USER_BASIC_AUTH.
  - Update the `enable_multi_user_basic_auth` field docstring to mark it
    as derived / not user-settable.
  - `_is_multi_user_mode()` (early-config helper, runs before Settings
    is built) switched to checking MCP_DEPLOYMENT_MODE directly. Now
    consistent with the canonical detection in detect_auth_mode.
- nextcloud_mcp_server/config_validators.py:
  - Drop the auto-detection branch (`if settings.enable_multi_user_basic_auth`).
    Selection of MULTI_USER_BASIC is now exclusively via the explicit
    MCP_DEPLOYMENT_MODE branch.
  - Add `enable_multi_user_basic_auth` to `_sync_derived_flags` alongside
    `enable_login_flow` — both flags are now derived from the resolved mode.
  - Drop `enable_multi_user_basic_auth` from
    `MODE_REQUIREMENTS[MULTI_USER_BASIC].required` and from the
    `forbidden` lists of SINGLE_USER_BASIC and LOGIN_FLOW (no longer
    user input → no meaningful forbidden check).
  - Add loud-deprecation `ValueError` block at the top of detect_auth_mode
    that errors with a clear migration message when ENABLE_MULTI_USER_BASIC_AUTH
    or ENABLE_LOGIN_FLOW is found in os.environ.
- tests/unit/test_config_validators.py:
  - Switch ~10 fixtures from `enable_multi_user_basic_auth=True` to
    `deployment_mode="multi_user_basic"` (mirrors `enable_login_flow`
    treatment from the previous commit).
  - Switch two `patch.dict(os.environ, {"ENABLE_MULTI_USER_BASIC_AUTH": "true"})`
    blocks to use MCP_DEPLOYMENT_MODE.
  - Rename `test_forbidden_multi_user_basic_auth` to
    `test_forbidden_multi_user_basic_when_credentials_present` — the
    scenario is now an explicit-mode + credentials conflict, not an
    env-var-flag conflict.
  - Add `test_legacy_enable_multi_user_basic_auth_env_var_errors` and
    `test_legacy_enable_login_flow_env_var_errors` to exercise the new
    loud-deprecation ValueError path.
- docker-compose.yml: mcp-multi-user-basic profile switched to
  `MCP_DEPLOYMENT_MODE=multi_user_basic`.
- env.sample: replaced `#ENABLE_MULTI_USER_BASIC_AUTH=true` example with
  `#MCP_DEPLOYMENT_MODE=multi_user_basic`.
- docs/authentication.md, configuration.md, troubleshooting.md,
  auth-flows.md, webhook-management-guide.md,
  configuration-migration-v2.md, ADR-025: replaced env-var examples
  with the canonical MCP_DEPLOYMENT_MODE form.
- docs/ADR-020: marked partly superseded by ADR-022.
- CLAUDE.md: Multi-User BasicAuth section updated to set
  MCP_DEPLOYMENT_MODE.
- nextcloud_mcp_server/vector/oauth_sync.py: module docstring updated.

BREAKING CHANGE: ENABLE_MULTI_USER_BASIC_AUTH is no longer read from
the environment, and setting it now raises a startup ValueError with
a migration message. Replace `ENABLE_MULTI_USER_BASIC_AUTH=true` with
`MCP_DEPLOYMENT_MODE=multi_user_basic`. The same loud-deprecation
check is also applied to the recently-removed ENABLE_LOGIN_FLOW —
replace with `MCP_DEPLOYMENT_MODE=login_flow` (or drop both;
`login_flow` is the auto-detect default when no other auth env vars
are set).

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

docs/configuration-migration-v2.md

@@ -188,7 +188,7 @@ NEXTCLOUD_OIDC_CLIENT_SECRET=secret
 **Before (v0.57.x):**
 ```bash
 NEXTCLOUD_HOST=https://nextcloud.example.com
-ENABLE_MULTI_USER_BASIC_AUTH=true
+MCP_DEPLOYMENT_MODE=multi_user_basic
 
 # Both required - redundant
 ENABLE_OFFLINE_ACCESS=true
@@ -205,7 +205,7 @@ NEXTCLOUD_OIDC_CLIENT_SECRET=secret
 **After (v0.58.0+ - Simplified):**
 ```bash
 NEXTCLOUD_HOST=https://nextcloud.example.com
-ENABLE_MULTI_USER_BASIC_AUTH=true
+MCP_DEPLOYMENT_MODE=multi_user_basic
 
 # Optional: Explicit mode declaration
 MCP_DEPLOYMENT_MODE=multi_user_basic
@@ -448,7 +448,7 @@ Server activates `login_flow` mode when you expected `multi_user_basic`
 Add explicit mode declaration:
 ```bash
 MCP_DEPLOYMENT_MODE=multi_user_basic
-ENABLE_MULTI_USER_BASIC_AUTH=true
+MCP_DEPLOYMENT_MODE=multi_user_basic
 ```
 
 ---