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>

src/physcom_web/templates/domains/list.html

@@ -13,12 +13,13 @@
     <p>{{ d.description }}</p>
     <table>
       <thead>
-        <tr><th>Metric</th><th>Weight</th><th>Norm Min</th><th>Norm Max</th></tr>
+        <tr><th>Metric</th><th>Unit</th><th>Weight</th><th>Norm Min</th><th>Norm Max</th></tr>
       </thead>
       <tbody>
         {% for mb in d.metric_bounds %}
         <tr>
           <td>{{ mb.metric_name }}</td>
+          <td>{{ mb.unit }}</td>
           <td>{{ mb.weight }}</td>
           <td>{{ mb.norm_min }}</td>
           <td>{{ mb.norm_max }}</td>