feat(telemetry): add MCP per-turn structured-log telemetry (Phase 0.5)

Emits structured `mcp.turn` log events on every Anthropic-path chat turn,
capturing whether MCP was wired in (mcp_available), whether the model
actually invoked an MCP tool (mcp_invoked), which tool names fired,
and whether the silent retry-without-MCP fallback was triggered.
Adds a separate `mcp.fallback` event with error type/message for
fallback occurrences.

Establishes baseline data for deciding whether MCP investment is earning
its keep before Phase 2+ expands the product footprint. Scope: the one
MCP-using code path (`_call_anthropic_cached`) — not a general
instrumentation layer.

No new dependencies, no schema changes, no behavior change. Standard
library `logging` is the sink; PostHog is not wired on the backend.

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

backend/app/services/assistant_chat_service.py

@@ -304,6 +304,7 @@ async def _call_anthropic_cached(
         ]
 
     _mcp_active = mcp_servers is not anthropic.NOT_GIVEN
+    _mcp_fallback_triggered = False
 
     try:
         response = await client.beta.messages.create(
@@ -326,10 +327,20 @@ async def _call_anthropic_cached(
             or isinstance(e, (anthropic.BadRequestError, anthropic.APIStatusError))
         )
         if _is_mcp_error:
+            _mcp_fallback_triggered = True
             logger.warning(
                 "MCP server error (%s), retrying without MCP: %s",
                 type(e).__name__, e,
             )
+            # Phase 0.5 telemetry: per-turn fallback event.
+            logger.info(
+                "mcp.fallback",
+                extra={
+                    "event": "mcp.fallback",
+                    "mcp_error_type": type(e).__name__,
+                    "mcp_error_message": str(e)[:500],
+                },
+            )
             response = await client.messages.create(
                 model=settings.AI_MODEL_ANTHROPIC,
                 max_tokens=max_tokens,
@@ -355,7 +366,23 @@ async def _call_anthropic_cached(
     input_tokens = usage.input_tokens
     output_tokens = usage.output_tokens
 
-    # Log MCP tool usage
+    # Phase 0.5 telemetry: per-turn MCP event. Emitted for every turn that
+    # reached this code path (i.e., AI_PROVIDER=anthropic chat). `mcp_available`
+    # reflects whether MCP was actually wired into the request (scope (ii) from
+    # the Phase 0.5 design — Anthropic code path AND flag on). `mcp_invoked`
+    # reflects whether the model chose to call an MCP tool on this turn.
+    logger.info(
+        "mcp.turn",
+        extra={
+            "event": "mcp.turn",
+            "mcp_available": _mcp_active,
+            "mcp_invoked": bool(mcp_tools_used),
+            "mcp_tools": mcp_tools_used,
+            "mcp_fallback_triggered": _mcp_fallback_triggered,
+        },
+    )
+
+    # Human-readable log retained for grep-based inspection.
     if mcp_tools_used:
         logger.info("MCP tools used: %s", ", ".join(mcp_tools_used))