perf(app): reduce redundant data fetching

Share bounded database pools across API repositories, consolidate sync and chat queries, and make mobile refreshes cache-aware and foreground-aware.

Add focused regression coverage, client freshness documentation, and repository agent role configuration.

.codex/agents/test-writer.toml

@@ -0,0 +1,11 @@
+name = "test_writer"
+description = "Focused Sauci regression and contract-test author for already-defined behavior."
+model = "gpt-5.6-terra"
+model_reasoning_effort = "medium"
+developer_instructions = """
+Read the applicable AGENTS.md and testing documentation. Test the intended
+observable contract at the lowest useful layer while preserving real auth,
+authorization, tenant, and couple boundaries. Do not weaken assertions to
+accommodate a failure. Run the focused test and return files changed, exact
+commands, results, and any product defect exposed by the test.
+"""