Display metric units in web UI, make seed idempotent, simplify code

- Load m.unit in get_domain() so MetricBound carries units from DB
- Add Unit column to domains list template
- Make load_transport_seed() idempotent with IntegrityError handling
  and metric unit backfill for existing DBs
- Remove unused imports (json, sqlite3, Entity)
- Simplify combinator loop to list comprehension
- Merge duplicate conditional/valid branches in pipeline
- Consolidate duplicated SQL in get_all_results()
- Expand CLAUDE.md with fuller architecture docs and conventions

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

tests/conftest.py

@@ -2,8 +2,6 @@
 
 from __future__ import annotations
 
-import sqlite3
-
 import pytest
 
 from physcom.db.schema import init_db

tests/test_pipeline_async.py

@@ -1,7 +1,5 @@
 """Tests for async pipeline: resume, cancellation, status guard, run lifecycle."""
 
-import json
-
 from physcom.engine.constraint_resolver import ConstraintResolver
 from physcom.engine.scorer import Scorer
 from physcom.engine.pipeline import Pipeline, CancelledError
@@ -287,7 +285,6 @@ def test_save_combination_loads_existing_status(seeded_repo):
     """save_combination should load the status of an existing combo from DB."""
     repo = seeded_repo
     from physcom.models.combination import Combination
-    from physcom.models.entity import Entity
 
     entities = repo.list_entities(dimension="platform")[:1] + repo.list_entities(dimension="power_source")[:1]
     combo = Combination(entities=entities)