#7 Ingest HTTPS endpoint: accept report POST, size limit

Add internal/ingest implementing POST /v1/reports, wired into the core
build-tag-free handler so the same route serves on the dev server and the
Cloudflare Worker.

Response contract (ADR #6 §2.4):
- 202 Accepted for valid JSON within the 256 KiB cap ({"status":"accepted"})
- 413 for oversized bodies (Content-Length fast path AND a MaxBytesReader
  hard cap, so a missing/lying Content-Length cannot bypass the limit)
- 415 when Content-Type is not application/json
- 400 for malformed JSON or failed schema validation (generic error body,
  never echoes request content)
- 405 with Allow: POST for any non-POST method
- 503 when the storage Sink fails

Storage is decoupled behind a small Sink interface (Store(ctx, raw)) with a
NopSink default and a MemorySink for tests, so PII scrubbing (#8) and
encrypted R2 storage (#9) can slot in without touching the HTTP contract.
Rate limiting (429) and volumetric shedding stay a Cloudflare-edge/Pulumi
concern per #2 and are intentionally not implemented in the Worker.

Tests:
- Go unit tests (net/http/httptest) for every response code, including 413
  via both Content-Length and an oversized streamed body, plus boundary,
  storage-failure, and no-content-echo cases.
- Bruno API tests in OpenCollection YAML format under api-tests/, asserting
  the full contract against the local dev server via @usebruno/cli.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

api-tests/wrong-method.yml

@@ -0,0 +1,19 @@
+info:
+  name: 405 - non-POST method is rejected
+  type: http
+  seq: 5
+
+http:
+  method: GET
+  url: "{{baseUrl}}/v1/reports"
+
+runtime:
+  scripts:
+    - type: tests
+      code: |-
+        test("a non-POST method returns 405", function () {
+          expect(res.getStatus()).to.equal(405);
+        });
+        test("405 advertises the allowed method via the Allow header", function () {
+          expect(res.getHeader("allow")).to.equal("POST");
+        });