feat(assistant): implement tool registry and expand assistant capabilities

Introduce a centralized tool registry system for the assistant to manage
executable operations, permissions, and tool catalogs. This refactors
the assistant dispatch logic from a monolithic approach to a modular,
decorator-based registry.

Key changes:
- Implement `AssistantToolRegistry` to handle tool registration,
  permission checks, and safe operation identification.
- Add a suite of new assistant tools: backup, commit, branch creation,
  deployment, health summary, environment listing, LLM operations,
  maintenance, migration, and dashboard searching.
- Refactor `_dispatch.py` and `_llm_planner.py` to utilize the new
  registry for intent resolution and tool catalog building.
- Enhance the LLM planner to support more descriptive clarification
  responses in Russian when intents are ambiguous.
- Update the frontend to support the new tool-based workflow, including
  improved i18n labels for assistant operations and a new step-by-step
  wizard for the migration page.
- Add ADR-0008 to document the architectural decision for the tool
  registry.

backend/src/api/routes/assistant/_llm_planner_intent.py

@@ -34,7 +34,7 @@ from ._resolvers import (
     _is_production_env,
     _resolve_provider_id,
 )
-from ._schemas import INTENT_PERMISSION_CHECKS
+from ._tool_registry import get_permission_checks
 
 
 # #region _plan_intent_with_llm [C:2] [TYPE Function]
@@ -71,7 +71,7 @@ async def _plan_intent_with_llm(
     )
 
     system_instruction = (
-        "You are a deterministic intent planner for backend tools.\n"
+        "You are a helpful assistant that understands backend tools.\n"
         "Choose exactly one operation from available_tools or return clarify.\n"
         "Output strict JSON object:\n"
         "{"
@@ -85,6 +85,8 @@ async def _plan_intent_with_llm(
         "Rules:\n"
         "- Use only operation names from available_tools.\n"
         '- If input is ambiguous, operation must be "clarify" with low confidence.\n'
+        '  Include "clarification" field with a specific question in Russian '
+        "asking what exactly is missing or unclear.\n"
         "- If dashboard is provided as name/slug (e.g., COVID), put it into entities.dashboard_ref.\n"
         "- Keep entities minimal and factual.\n"
     )
@@ -115,6 +117,7 @@ async def _plan_intent_with_llm(
     operation = response.get("operation")
     valid_ops = {tool["operation"] for tool in tools}
     if operation == "clarify":
+        clarification = response.get("clarification", "")
         return {
             "domain": "unknown",
             "operation": "clarify",
@@ -122,6 +125,7 @@ async def _plan_intent_with_llm(
             "confidence": float(response.get("confidence", 0.3)),
             "risk_level": "safe",
             "requires_confirmation": False,
+            "clarification": clarification,
         }
     if operation not in valid_ops:
         return None
@@ -164,8 +168,9 @@ async def _plan_intent_with_llm(
 # @POST  Returns if authorized; raises HTTPException(403) when denied.
 def _authorize_intent(intent: dict[str, Any], current_user: User):
     operation = intent.get("operation")
-    if operation in INTENT_PERMISSION_CHECKS:
-        _check_any_permission(current_user, INTENT_PERMISSION_CHECKS[operation])
+    checks = get_permission_checks().get(operation)
+    if checks:
+        _check_any_permission(current_user, checks)
 
 
 # #endregion _authorize_intent