feat(backend): add LLM-powered feed enhancement

Add enhancement harness that fills feed item slots and
generates synthetic items via OpenRouter.

- LLM client with 30s timeout, reusable SDK instance
- Prompt builder with mini calendar and week overview
- arktype schema validation + JSON Schema for structured output
- Pure merge function with clock injection
- Defensive fallback in feed endpoint on enhancement failure
- Skips LLM call when no unfilled slots or no API key

Co-authored-by: Ona <no-reply@ona.com>

bun.lock

@@ -18,9 +18,12 @@
       "version": "0.0.0",
       "dependencies": {
         "@aris/core": "workspace:*",
+        "@aris/source-caldav": "workspace:*",
+        "@aris/source-google-calendar": "workspace:*",
         "@aris/source-location": "workspace:*",
         "@aris/source-tfl": "workspace:*",
         "@aris/source-weatherkit": "workspace:*",
+        "@openrouter/sdk": "^0.9.11",
         "arktype": "^2.1.29",
         "better-auth": "^1",
         "hono": "^4",
@@ -543,6 +546,8 @@
 
     "@oclif/screen": ["@oclif/screen@3.0.8", "", {}, "sha512-yx6KAqlt3TAHBduS2fMQtJDL2ufIHnDRArrJEOoTTuizxqmjLT+psGYOHpmMl3gvQpFJ11Hs76guUUktzAF9Bg=="],
 
+    "@openrouter/sdk": ["@openrouter/sdk@0.9.11", "", { "dependencies": { "zod": "^3.25.0 || ^4.0.0" } }, "sha512-BgFu6NcIJO4a9aVjr04y3kZ8pyM71j15I+bzfVAGEvxnj+KQNIkBYQGgwrG3D+aT1QpDKLki8btcQmpaxUas6A=="],
+
     "@oxfmt/darwin-arm64": ["@oxfmt/darwin-arm64@0.24.0", "", { "os": "darwin", "cpu": "arm64" }, "sha512-aYXuGf/yq8nsyEcHindGhiz9I+GEqLkVq8CfPbd+6VE259CpPEH+CaGHEO1j6vIOmNr8KHRq+IAjeRO2uJpb8A=="],
 
     "@oxfmt/darwin-x64": ["@oxfmt/darwin-x64@0.24.0", "", { "os": "darwin", "cpu": "x64" }, "sha512-vs3b8Bs53hbiNvcNeBilzE/+IhDTWKjSBB3v/ztr664nZk65j0xr+5IHMBNz3CFppmX7o/aBta2PxY+t+4KoPg=="],