fix: delete orphaned digest directories when last tag is removed

[hiroTamada]

Summary

• Fix orphaned digest directory accumulation by implementing eager garbage collection
• When DeleteImage removes the last tag referencing a digest, the digest directory is now automatically deleted
• If multiple tags share the same digest, it's preserved until the last tag is removed

Background

Previously, DeleteImage only removed the tag symlink, leaving digest directories (containing erofs rootfs files) on disk indefinitely. This was documented in the README as "Old digests remain until explicitly garbage collected" but no GC was ever implemented.

At current volume (~20 GB/month), the 3.5 TB disk on prod-iad-hypeman-1 has runway, but this becomes a concern at higher deployment volumes post-rollout.

Changes

• storage.go: Added countTagsForDigest() and deleteDigest() helper functions
• manager.go: Updated DeleteImage() to check for orphaned digests and delete them
• manager_test.go: Updated existing test + added TestDeleteImagePreservesSharedDigest
• README.md: Updated documentation to reflect new behavior

CI fixes (pre-existing issues on main)

• go.mod: Updated oapi-codegen/runtime v1.1.2 → v1.2.0 (fixes missing StyleParamWithOptions)
• Makefile: Pinned oapi-codegen to v2.5.1 (v2.6.0 generates incompatible struct tags)

Test plan

• TestDeleteImage - verifies digest is deleted when orphaned
• TestDeleteImagePreservesSharedDigest - verifies digest is preserved when other tags reference it
• TestDeleteImageNotFound - unchanged, still passes
• Manual test on staging: deploy, stop, verify disk space is reclaimed

Note on CI fixes

This PR includes fixes for two pre-existing CI issues unrelated to the GC changes:

1. Runtime version: CI regenerates oapi.go with oapi-codegen@latest, which uses runtime.StyleParamWithOptions added in v1.2.0, but go.mod had v1.1.2.

2. Generator version: oapi-codegen v2.6.0 changed struct tag generation (adds omitempty to some fields), causing type mismatches. Pinned to v2.5.1 to match committed code.

Both issues caused CI failures starting with commit 08958b8 on main.

---

Note

Medium Risk
Changes deletion semantics for on-disk image storage by removing digest directories automatically, with added locking to avoid races; bugs here could delete still-referenced data or leave dangling tags under concurrency.

Overview
Image deletion now performs eager cleanup. DeleteImage resolves the tag’s digest, removes the tag symlink, then (under createMu) counts remaining tags pointing at that digest and deletes the digest directory when it becomes orphaned.

Storage helpers + tests/documentation updated. Adds countTagsForDigest/deleteDigest, updates tests to assert digest dir removal and adds a shared-digest preservation test, and updates the images README to reflect the new GC behavior.

Build/CI stability tweaks. Pins oapi-codegen in the Makefile and bumps github.com/oapi-codegen/runtime to v1.2.0 to match generated code expectations.

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