feat: add post3 s3 proxy for postgresql

Signed-off-by: kjuulh <contact@kjuulh.io>

todos/POST3-013-s3-compliance.md

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+# POST3-013: S3 Compliance Testing with Ceph s3-tests
+
+## Status: Done
+
+## Summary
+
+Integrate Ceph s3-tests (the industry-standard S3 conformance suite) to validate post3's S3 compatibility. Uses the filesystem backend (`--backend fs`) for fast, database-free test runs.
+
+## Results
+
+**124 tests passing, 0 failures, 0 errors** out of 829 total tests (705 deselected for unimplemented features).
+
+## What was done
+
+### Phase 1 — Missing S3 operations (blocking for s3-tests)
+- [x] `ListObjectVersions` stub — `GET /{bucket}?versions` (returns objects as version "null")
+- [x] `DeleteObjects` batch delete — `POST /{bucket}?delete`
+- [x] `ListObjects` v1 — `GET /{bucket}` without `list-type=2`
+- [x] `GetBucketLocation` — `GET /{bucket}?location`
+- [x] `--backend fs/pg` CLI flag + `--data-dir`
+- [x] Bucket naming validation (S3 rules: 3-63 chars, lowercase, no IP format)
+
+### Phase 2 — Delimiter & listing compliance
+- [x] Delimiter + CommonPrefixes in `list_objects_v2` (both backends)
+- [x] V1 and V2 XML responses emit delimiter/common_prefixes
+- [x] MaxKeys limits total objects+common_prefixes combined (sorted interleave)
+- [x] MaxKeys=0 returns empty, non-truncated result
+- [x] StartAfter + ContinuationToken echo in v2 response
+- [x] Owner element in v1 Contents
+- [x] Empty delimiter treated as absent
+
+### Phase 3 — Test infrastructure
+- [x] s3-tests git submodule (pinned at `06e2c57`)
+- [x] `s3-compliance/s3tests.conf.template`
+- [x] `s3-compliance/run-s3-tests.sh`
+- [x] mise tasks: `test:s3-compliance` and `test:s3-compliance:dry`
+
+### Phase 4 — Compliance fixes from test runs
+- [x] ETag quoting normalization in multipart completion (both backends)
+- [x] ListObjectVersions pagination (NextKeyMarker/NextVersionIdMarker when truncated)
+- [x] ListObjectVersions passes key-marker and delimiter from query params
+- [x] EntityTooSmall validation (non-last parts must be >= 5 MB)
+- [x] DeleteObjects 1000 key limit
+- [x] delete_object returns 404 for non-existent bucket
+- [x] Common prefix filtering by continuation token
+
+## Usage
+
+```sh
+mise run test:s3-compliance        # run filtered s3-tests
+mise run test:s3-compliance:dry    # list which tests would run
+```
+
+## Excluded test categories
+
+Features post3 doesn't implement (excluded via markers/keywords):
+ACLs, bucket policy, encryption, CORS, lifecycle, versioning, object lock,
+tagging, S3 Select, S3 website, IAM, STS, SSE, anonymous access, presigned URLs,
+CopyObject, logging, notifications, storage classes, auth signature validation,
+Range header, conditional requests, public access block.
+
+## Future work
+
+- [ ] Add CI step (`ci/src/main.rs`) for automated s3-compliance runs
+- [ ] Gradually reduce exclusion list as more features are implemented
+- [ ] Range header support (would enable ~10 more tests)
+- [ ] CopyObject support (would enable ~20 more tests)
+- [ ] Idempotent CompleteMultipartUpload (Ceph-specific, 2 excluded tests)