docs: pivot to Login Flow v2; add Astrolabe Cloud hosted offering

Replace the seven OAuth-to-Nextcloud docs (oauth-setup, quickstart-oauth,
oauth-architecture, oauth-upstream-status, oauth-troubleshooting,
jwt-oauth-reference, audience-validation-setup) with a single new
docs/login-flow-v2.md. The deprecated flow required upstream user_oidc
patches that were never merged; Login Flow v2 is the forward-looking
multi-user mode (see ADR-022), and works with stock Nextcloud 16+.

Rewrite docs/authentication.md and docs/auth-flows.md around three modes:
Single-User BasicAuth, Multi-User BasicAuth pass-through, and Login Flow v2.

Update README to add an Astrolabe Cloud (https://astrolabecloud.com)
callout for users who prefer not to self-host, drop the OAuth deployment
mode from the auth table, simplify the Docker block, and trim the
Examples and Security sections.

Sweep configuration.md, installation.md, troubleshooting.md, running.md,
and semantic-search-architecture.md to replace links to the deleted docs
and update deprecated mode names.

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

docs/installation.md

@@ -132,7 +132,7 @@ docker-compose up -d
 After installation:
 
 1. **Configure the server** - See [Configuration Guide](configuration.md)
-2. **Set up authentication** - See [OAuth Setup Guide](oauth-setup.md) or [Authentication](authentication.md)
+2. **Set up authentication** - See [Authentication](authentication.md) (multi-user deployments: see [Login Flow v2](login-flow-v2.md))
 3. **Run the server** - See [Running the Server](running.md)
 
 ## Updating
@@ -211,5 +211,6 @@ sudo docker run ...
 ## See Also
 
 - [Configuration Guide](configuration.md) - Environment variables and settings
-- [OAuth Setup Guide](oauth-setup.md) - OAuth authentication setup
+- [Authentication](authentication.md) - Authentication modes comparison
+- [Login Flow v2](login-flow-v2.md) - Recommended multi-user setup
 - [Running the Server](running.md) - Starting and managing the server