Remove obsolete [1m] model variants

Since March 2026 the 1M-token context window is the default for Opus 4.6/4.7
and Sonnet 4.6 – no separate variant or beta header needed. The "[1m]" suffix
on model IDs is dead code now and would even produce HTTP 400 against the
current API.

Cleanup:
- Drop 3 [1m] options from the agent model dropdown; default selection moves
  from opus-4-7[1m] to opus-4-7.
- Drop 5 [1m] mappings from ResolveModelId and add a defensive Replace as a
  safety net for any agent record still holding an old value.
- One-shot startup migration on agents.model + chains.model strips the suffix
  so existing chains can be re-chatted without errors.
- ARCHITECTURE.md reflects the slimmed list and explains why 1M is now default.

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

Services/ChatService.cs

@@ -34,20 +34,20 @@ public class ChatService
         // OpenAI-kompatible Provider werden unverändert durchgereicht (Format: "provider:model")
         if (OpenAiService.IsOpenAiProvider(model)) return model;
 
+        // Defensiv-Strip: alte "[1m]"-Suffixe aus DB transparent normalisieren.
+        // 1M-Context ist seit März 2026 Default bei Opus 4.6/4.7/4.8 + Sonnet 4.6 – kein Suffix mehr nötig.
+        if (!string.IsNullOrEmpty(model) && model.Contains("[1m]"))
+            model = model.Replace("[1m]", "");
+
         return model switch
         {
             "sonnet" => "claude-sonnet-4-6",
-            "sonnet[1m]" => "claude-sonnet-4-6[1m]",
             "sonnet-4-5" => "claude-sonnet-4-5",
             "sonnet-4-6" => "claude-sonnet-4-6",
-            "sonnet-4-6[1m]" => "claude-sonnet-4-6[1m]",
             "opus" => "claude-opus-4-7",
-            "opus[1m]" => "claude-opus-4-7[1m]",
             "opus-4-5" => "claude-opus-4-5",
             "opus-4-6" => "claude-opus-4-6",
-            "opus-4-6[1m]" => "claude-opus-4-6[1m]",
             "opus-4-7" => "claude-opus-4-7",
-            "opus-4-7[1m]" => "claude-opus-4-7[1m]",
             "haiku" => "claude-haiku-4-5",
             "haiku-4-5" => "claude-haiku-4-5",
             _ => "claude-sonnet-4-6"