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/seed/transport_example.py

@@ -265,22 +265,35 @@ INTERPLANETARY = Domain(
 
 
 def load_transport_seed(repo) -> dict:
-    """Load all transport seed data into the repository. Returns counts."""
+    """Load all transport seed data into the repository. Idempotent — safe to re-run."""
+    import sqlite3
     from physcom.db.repository import Repository
     repo: Repository
 
     counts = {"platforms": 0, "power_sources": 0, "domains": 0}
 
     for entity in PLATFORMS:
-        repo.add_entity(entity)
-        counts["platforms"] += 1
+        try:
+            repo.add_entity(entity)
+            counts["platforms"] += 1
+        except sqlite3.IntegrityError:
+            pass
 
     for entity in POWER_SOURCES:
-        repo.add_entity(entity)
-        counts["power_sources"] += 1
+        try:
+            repo.add_entity(entity)
+            counts["power_sources"] += 1
+        except sqlite3.IntegrityError:
+            pass
 
-    repo.add_domain(URBAN_COMMUTING)
-    repo.add_domain(INTERPLANETARY)
-    counts["domains"] = 2
+    for domain in (URBAN_COMMUTING, INTERPLANETARY):
+        try:
+            repo.add_domain(domain)
+            counts["domains"] += 1
+        except sqlite3.IntegrityError:
+            pass
+        # Backfill metric units on existing DBs (ensure_metric is idempotent).
+        for mb in domain.metric_bounds:
+            repo.ensure_metric(mb.metric_name, unit=mb.unit)
 
     return counts