perf(task-board): delete PR cache keys concurrently on invalidate

[pedrofrxncx]

Source: reduction/optimization found while hunting the mcp-tool-latency-profiling focus area (relates to #2995 "DB latency amplifies significantly across MCP tool calls") — read pr-cache.ts (added by #6966, the JetStream KV cache fronting task-board PR reads/cards).

The problem: JetStreamKVPrCache.invalidate(namespace) lists every KV key under a namespace, then deletes them one at a time in a for await loop — each kv.delete() is its own NATS round-trip, awaited sequentially. invalidatePrCards(organizationId) and invalidatePrReadsForConnection(connectionId) are both awaited synchronously from task-board tool handlers (e.g. TASK_BOARD_ITEM_PR_LINK, merge-pr) right before they return. An org or connection with many cached PR entries pays N sequential round-trips inline on that tool call — the busier the board, the slower every PR-link/merge call gets, which is exactly the tool-call latency amplification pattern #2995 describes.

The fix: collect the matched keys, then Promise.all the deletes. They're independent, unrelated keys with no ordering requirement, and the existing per-key .catch(() => {}) (best-effort, TTL is the backstop) is preserved — behavior is unchanged, just concurrent instead of serial.

Verify: cd apps/api && bun test src/tools/task-board/pr-cache.test.ts — added a test asserting invalidate still drops every key under a namespace (not just the first), which would have caught an incorrect early-return/short-circuit in a naive rewrite.

Checks run locally: bun run fmt, bunx tsc --noEmit (apps/api), the targeted test above, and bunx oxlint on both changed files — all clean. Full CI runs the rest.

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Summary by cubic

Speeds up task-board PR cache invalidation by deleting matched keys concurrently instead of one at a time, cutting the serial NATS round-trips that slow PR-link and merge tool calls on busy boards (the latency pattern in #2995). Behavior is unchanged — deletes are independent and best-effort with TTL as the backstop.

• Switches invalidate() from a sequential for await loop to Promise.all over collected keys.
• Adds a test asserting all keys under a namespace are dropped, not just the first.

^{Written for commit 254a341. Summary will update on new commits.}