test: add self-contained integration environment

Separate schema migrations from environment-specific fixtures, run the full API workflow against a disposable PostgreSQL Testcontainer, and provide seeded development and UI test environments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/internal/database/seed/common.sql

@@ -0,0 +1,9 @@
+INSERT INTO traits (name, normalized) VALUES
+    ('Go', 'go'),
+    ('PostgreSQL', 'postgresql'),
+    ('Docker', 'docker'),
+    ('React', 'react'),
+    ('TypeScript', 'typescript'),
+    ('Kubernetes', 'kubernetes'),
+    ('Backend', 'backend')
+ON CONFLICT (normalized) DO NOTHING;