feat(db): enforce globally unique issue prefixes and identifiers

Derive issue_prefix from first 3 letters of company name with
deterministic suffixes on collision. Migration rebuilds existing
prefixes, reassigns issue numbers, and adds unique indexes.

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

packages/db/src/migrations/meta/_journal.json

@@ -120,6 +120,13 @@
       "when": 1771955900000,
       "tag": "0016_agent_icon",
       "breakpoints": true
+    },
+    {
+      "idx": 17,
+      "version": "7",
+      "when": 1771883888199,
+      "tag": "0017_tiresome_gabe_jones",
+      "breakpoints": true
     }
   ]
 }