feat(ai): streaming chat endpoint with live tool events

Add LlmClient::chat_with_tools_stream and SSE endpoint
POST /insights/chat/stream that emits text deltas, tool_call /
tool_result pairs, truncated notice, and a terminal done frame as the
agentic loop runs.

- Ollama: parses NDJSON from /api/chat stream, accumulates content
  deltas, emits Done with tool_calls from the final chunk.
- OpenRouter: parses OpenAI-compatible SSE, reassembles tool_call
  argument deltas by index, asks for stream_options.include_usage.
- InsightChatService spawns the loop on a tokio task, feeds events
  through an mpsc channel, persists training_messages at the end.

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

Cargo.toml

@@ -49,7 +49,10 @@ opentelemetry-appender-log = "0.31.0"
 tempfile = "3.20.0"
 regex = "1.11.1"
 exif = { package = "kamadak-exif", version = "0.6.1" }
-reqwest = { version = "0.12", features = ["json"] }
+reqwest = { version = "0.12", features = ["json", "stream"] }
+async-stream = "0.3"
+tokio-util = { version = "0.7", features = ["io"] }
+bytes = "1"
 urlencoding = "2.1"
 zerocopy = "0.8"
 ical = "0.11"