Add async pipeline with progress monitoring, resumability, and result transparency

Pipeline engine rewritten with combo-first loop: each combination is processed
through all requested passes before moving to the next, with incremental DB
saves after every step (crash-safe). Blocked combos now get result rows so they
appear in the results page with constraint violation reasons.

New pipeline_runs table tracks run lifecycle (pending/running/completed/failed/
cancelled). Web route launches pipeline in a background thread with its own DB
connection. HTMX polling partial shows live progress with per-pass breakdown.

Also: status guard prevents reviewed->scored downgrade, save_combination loads
existing status on dedup for correct resume, per-metric scores show domain
bounds + units + position bars, ensure_metric backfills units on existing rows.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/physcom/models/domain.py

@@ -13,6 +13,7 @@ class MetricBound:
     weight: float  # 0.0–1.0
     norm_min: float  # Below this → score 0
     norm_max: float  # Above this → score 1
+    unit: str = ""
     metric_id: int | None = None