fix: Streaming assistant snapshots rewrite FTS rows on every upsert

[sam-saffron-jarvis]

What changed

• Added a streaming-aware message update path in internal/session so interim assistant snapshot updates can skip rewriting messages.text_content until the assistant message is finalized.
• Narrowed the SQLite FTS update trigger from AFTER UPDATE ON messages to AFTER UPDATE OF text_content ON messages, plus added a schema migration so existing databases pick up the same behavior.
• Updated the hot streaming persistence call sites in both cmd/serve_runtime.go and internal/tui/chat/streaming.go to:
  • persist ordinary snapshots without touching FTS-backed text,
  • persist final assistant text once on response completion, and
  • avoid a duplicate final FTS rewrite when turn completion follows response completion.
• Added a regression test proving snapshot updates no longer surface in FTS until finalization, while the final assistant text is still searchable.

Why this is high-value

This path is hit repeatedly during real streaming turns in both the web runtime and TUI. Before this change, every snapshot update rewrote the entire FTS row because UpdateMessage always updated text_content and the messages_au trigger reindexed on every update.

The fix reduces work in the hot path from "reindex the growing assistant reply on every snapshot" to "update non-FTS message fields during snapshots, then index the final text once". That cuts SQLite write amplification, reduces lock contention, and improves responsiveness on long assistant replies and tool-heavy turns without changing final search behavior.

Validation

• Verified the issue in current code by tracing UpdateMessage updating text_content on every snapshot and the unconditional messages_au FTS trigger.
• Added TestSQLiteStoreUpdateStreamingMessageDefersFTSRewriteUntilFinalized to cover the exact regression.
• Ran:
  • gofmt -w cmd/serve_runtime.go internal/session/store.go internal/session/logger.go internal/session/sqlite.go internal/session/update_message_test.go internal/tui/chat/chat.go internal/tui/chat/streaming.go
  • go build ./...
  • go test ./...
  • git diff --check

[SamSaffron]

Implemented directly in main working tree with the same fixes and regression coverage.