Clamp an unlimited download queue's LIMIT for Postgres (#98)

With max_new_per_check at 0 and no per-subscription limit, the budget is usize::MAX, and
pending() bound it `as i64`: -1. SQLite reads LIMIT -1 as no limit; Postgres refuses it, so a
feed's downloads failed. The new test fails with "LIMIT must not be negative" on Postgres
without the clamp and passes with it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.claude/settings.local.json

@@ -1,14 +0,0 @@
-{
-  "permissions": {
-    "allow": [
-      "Bash(rtk grep *)",
-      "Bash(rtk read *)",
-      "Bash(rtk git *)"
-    ],
-    "additionalDirectories": [
-      "/config/.claude/skills/security-audit",
-      "/config/security-audit-skill",
-      "/config/.cargo/registry"
-    ]
-  }
-}