test: add identity property tests and full-chain otlp e2e

[AKogut]

Closes #103

Summary

Closes the last review gap: #18 asked for property tests (only example-based existed) and no single test exercised the whole write path — every stage was tested in isolation.

Changes

• Property tests (fast-check, 7 identity invariants): a moved file resolves to the same identity for any pair of paths; a changed title is a different identity even at the same path; resolution works through an alias recorded by an earlier move; normalizeFilePath is idempotent; hashParams is key-order independent; parameterized variants stay distinct.
• Full-chain E2E (2 tests, real Postgres): the SDK exports real OTLP to a live API server → durable queue → worker → asserts the resulting rows (run, executions with retry linkage, identity, flaky score with reason codes) and the emitted run.processed event. The second test covers history accumulating across runs and idempotency on re-delivery.
• buildApp is now a separate @flakemetry/api/app entry so it can be imported without side effects (importing index.ts used to start the server).

A flaky test, caught and fixed properly

The first version of the E2E was itself flaky: a single tick() occasionally missed a just-enqueued job. Investigated rather than retried — this is not a production bug, because the real worker polls in a loop and would pick the job up on the next poll. The fix was to the test design (a bounded drain loop mirroring real worker behaviour), not a retry papering over the race. Stable across repeated runs.

Testing

• core: 22 tests (7 new property tests)
• worker: 11 tests (2 new E2E against real Postgres)
• full turbo pipeline green, 38/38 tasks