slice 6: LLM + Chromium PDF renderer worker

Watches mission_artifacts for MD entries with render_pdf_status='pending'
and turns them into styled PDFs via:
  1. Read source MD from <mission_root>/<path>
  2. Call configured LLM (default gemini-2.5-flash) with a document-
     typesetter system prompt that constrains style to a self-contained
     HTML doc with inline CSS + our color palette
  3. Print to PDF via `chromium --headless=new --print-to-pdf`
  4. Save alongside source MD (foo.md → foo.pdf) + update
     mission_artifacts.rendered_pdf_path + render_pdf_status='done'

Graceful degradation: GEMINI_API_KEY unset OR chromium missing =
row marked failed with a descriptive error, worker keeps ticking.
The frontend's "Open PDF" affordance (Slice 2) light up automatically
when render succeeds.

Boot ordering: PDF worker spawns after task_card_worker. Poll every
30s over up to MAX_PARALLEL=2 rows at a time — respects LLM rate
limits and keeps chromium's peak RAM under control.

Env knobs:
  GEMINI_API_KEY                    — required for LLM step
  CLAWMATES_PDF_RENDERER_MODEL      — model id, default gemini-2.5-flash
  CHROMIUM_BIN                      — chromium binary, default `chromium`
  CLAWMATES_MISSIONS_ROOT           — artifact dir root, default /var/lib/clawmates-missions

Dockerfile now installs chromium + fonts-liberation and sets
CHROMIUM_BIN=/usr/bin/chromium so the container image has everything
the renderer needs.

Also bumps workspace tokio deps to include the `process` feature
(required for tokio::process::Command).

Follow-ups:
  - Anthropic + OpenAI provider variants (only Gemini in this slice)
  - SSE stream on /api/missions/{id}/artifacts for the "PDF ready"
    notification instead of poll-via-mission-GET
  - Per-template PDF style overrides (currently one house style
    for all missions)

Co-Authored-By: Claude Opus 4.7 <[email protected]>

crates/bins/clawmates-server/src/main.rs

@@ -288,6 +288,12 @@ async fn run() -> Result<(), String> {
     // for INT-XX markers in event payloads and upserts mission_tasks
     // rows so the canvas renders a live status timeline.
     cm_api::task_card_worker::spawn(pool.clone());
+    // PDF renderer worker (Slice 6): watches mission_artifacts for
+    // MD entries with render_pdf_status='pending', calls the
+    // configured LLM (default Gemini 2.5 Flash) for styled HTML,
+    // prints to PDF via chromium --headless. No-op-friendly when
+    // GEMINI_API_KEY / chromium binary aren't configured.
+    cm_api::pdf_renderer::spawn(pool.clone());
     // Outbound-email delivery: drains the §15-gated `outbox` over SMTP. Inert
     // until CLAWMATES_SMTP_* is set, so it ships safely before credentials exist.
     cm_runtime::spawn_drainer(pool.clone(), std::time::Duration::from_secs(10));