Fix Paperclip integration (identifier→issue_prefix) + add settings page

- Fix column name mismatch in paperclip_client.py and app.py: Paperclip's
  companies table uses `issue_prefix`, not `identifier`
- Fix _LEGAL_DB_URL to read from POSTGRES_URL env var (used in container)
- Add settings page (/settings) for managing tag → Paperclip company mappings
- Replace "תיק חדש" nav item with "הגדרות" (new case is on home page)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/app.py

@@ -2113,9 +2113,9 @@ async def api_paperclip_companies():
         conn = await asyncpg.connect(pc_url)
         try:
             rows = await conn.fetch(
-                "SELECT id, name, identifier FROM companies ORDER BY name"
+                "SELECT id, name, issue_prefix FROM companies ORDER BY name"
             )
-            return [{"id": str(r["id"]), "name": r["name"], "identifier": r.get("identifier", "")} for r in rows]
+            return [{"id": str(r["id"]), "name": r["name"], "prefix": r.get("issue_prefix", "")} for r in rows]
         finally:
             await conn.close()
     except Exception as e: