This change adds support for image attachments to chat via add button
and clipboard paste. Files are stored in a new `chat_files` table and
referenced by ID in message content. File data is resolved from storage
at LLM dispatch time, keeping the message content column small.
Upload validates MIME types via content type or content sniffing against
an allowlist (png, jpeg, gif, webp). The retrieval endpoint serves files
with immutable caching headers. On the frontend, uploads start eagerly
on attach with a background fetch to pre-warm the browser HTTP cache so
the timeline renders instantly after send.
## Problem
Users hit this error when agent tool results contain Unicode null
characters:
```
persist step: insert tool result: pq: unsupported Unicode escape sequence
```
PostgreSQL's `jsonb` type rejects `\u0000` (Unicode null, U+0000) with
that error, even though it's valid JSON per RFC 8259. Tool results from
agents can contain this sequence — e.g. binary data, C-style strings, or
certain API responses.
## Root cause
`MarshalToolResult` and `MarshalContent` in `chatprompt.go` serialize
content blocks to JSON and pass them directly to `InsertChatMessage`
which casts to `::jsonb`. Go's `json.Marshal` / `json.Valid` accept
`\u0000`, but Postgres does not.
## Fix
Added `sanitizeJSONForPG()` which strips `\u0000` escape sequences from
serialized JSON before insertion. Uses `bytes.Contains` as a fast-path
check to avoid allocation when no null bytes are present (the common
case).
Applied to both `MarshalContent` (assistant messages) and
`MarshalToolResult` (tool result messages).
Mathias Fredriksson