docs: add SPEC.md with repository model and caching requirements

SPEC.md

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+# ArtifactAPI Specification
+
+## Repository model
+
+Every repository entry in `remotes.yaml` has two orthogonal fields:
+
+| field | values | meaning |
+|---|---|---|
+| `type` | `local`, `remote`, `virtual` | repository kind — how the repo is served |
+| `package` | `docker`, `rpm`, `alpine`, `generic` | package format — what protocol and caching rules to apply |
+
+**type**
+
+- `local` — files are uploaded directly to the API and stored in S3; no upstream.
+- `remote` — proxies and caches content from an upstream URL (`base_url`).
+- `virtual` — aggregates multiple repositories (not yet implemented).
+
+**package**
+
+- `docker` — upstream speaks the OCI Distribution API (Bearer auth, manifest/blob paths).
+- `rpm` — upstream is an RPM repository; repodata files are index files.
+- `alpine` — upstream is an Alpine APK repository; `APKINDEX.tar.gz` is an index file.
+- `generic` — plain HTTP file download; no format-specific logic.
+
+---
+
+## Caching
+
+Two cache classes determine retention:
+
+| class | stored | TTL |
+|---|---|---|
+| **file** | S3 object, no Redis entry | `file_ttl` — `0` means indefinite |
+| **index** | S3 object + Redis TTL key | `index_ttl` — when the Redis key expires the S3 object is deleted and re-fetched |
+
+Index files are mutable metadata that must expire. File-class objects are treated as immutable and cached indefinitely (unless `file_ttl` is set).
+
+---
+
+## Docker package rules
+
+### URL construction
+
+Remote URLs are prefixed with `/v2/` for `package: docker` remotes:
+
+```
+{base_url}/v2/{path}
+```
+
+e.g. `library/nginx/manifests/latest` → `https://registry-1.docker.io/v2/library/nginx/manifests/latest`
+
+### Authentication
+
+Docker registries use Bearer token challenges. On a `401 Unauthorized` response, the API:
+
+1. Parses the `WWW-Authenticate: Bearer` header for `realm`, `service`, and `scope`.
+2. Fetches a token from the auth realm, supplying `username`/`password` from the remote config if present.
+3. Retries the request with `Authorization: Bearer <token>`.
+
+Tokens are cached in-memory keyed by `(realm, service, scope, username)` and expire 30 seconds before their stated `expires_in`.
+
+### Cache classification
+
+| path pattern | mutable | class | TTL source |
+|---|---|---|---|
+| `/manifests/<tag>` | yes | index | `index_ttl` |
+| `/tags/list` | yes | index | `index_ttl` |
+| `/manifests/sha256:<digest>` | no | file | `file_ttl` |
+| `/blobs/sha256:<digest>` | no | file | `file_ttl` |
+
+Tag-based manifests and tag lists are mutable and cached as index. Digest-pinned manifests and blobs are content-addressed and cached indefinitely as files.
+
+### Blob deduplication
+
+Blobs are stored under a digest-keyed path shared across all images on the same remote:
+
+```
+{remote_name}/blobs/sha256/{digest}
+```
+
+The same layer pulled by different images is stored once.
+
+### Accept headers
+
+| path | `Accept` header sent upstream |
+|---|---|
+| `/manifests/…` | `application/vnd.docker.distribution.manifest.v2+json`, `application/vnd.oci.image.manifest.v1+json`, `application/vnd.oci.image.index.v1+json`, `application/vnd.docker.distribution.manifest.list.v2+json` |
+| `/blobs/…` | `application/octet-stream` |
+
+---
+
+## OCI Distribution API endpoint
+
+The API exposes a native Docker registry interface so clients can use `docker pull` directly:
+
+```
+GET  /v2/                                   — version ping
+GET  /v2/{remote}/{image}/manifests/{ref}   — fetch manifest
+HEAD /v2/{remote}/{image}/manifests/{ref}   — manifest metadata
+GET  /v2/{remote}/{image}/blobs/{digest}    — fetch blob
+HEAD /v2/{remote}/{image}/blobs/{digest}    — blob metadata
+```
+
+Responses include `Docker-Distribution-Api-Version`, `Docker-Content-Digest`, and the correct OCI `Content-Type` (detected from the manifest `mediaType` field).
+
+Only remotes with `package: docker` are accessible via this endpoint. All other remotes return `400`.
+
+---
+
+## include_patterns
+
+`include_patterns` is a list of Python regexes applied to every request before any upstream fetch or cache lookup.
+
+**Generic remotes (`/api/v1/remote/…`):**
+- Patterns match against the file path and the full path.
+- Index files (mutable metadata) bypass pattern checks and are always allowed.
+
+**Docker remotes (`/v2/…`):**
+- Patterns match against the image name (first two path segments, e.g. `library/nginx`) and the full path.
+- The index-file exemption does **not** apply — patterns restrict whole images, including their manifests and tag lists.
+- No patterns configured → all images allowed.
+
+Returns `403` when a request is blocked.
+
+---
+
+## Versioning
+
+The package version is derived from git tags via `hatch-vcs`. Tags follow the format `v{MAJOR}.{MINOR}.{PATCH}`.
+
+Docker images are built with the version injected at build time:
+
+```
+SETUPTOOLS_SCM_PRETEND_VERSION=<version> uv sync --frozen
+```
+
+The `Makefile` provides `patch`, `minor`, and `major` targets that tag the current commit and rebuild the container image.