feat: rename include/index patterns to immutable/mutable with per-remote TTL

Replace the include_patterns/index_patterns split with a clearer
immutable_patterns/mutable_patterns model:

- immutable_patterns: artifacts cached indefinitely (no TTL)
- mutable_patterns: artifacts that expire and are re-fetched after
  cache.mutable_ttl seconds (replaces cache.index_ttl)

_PACKAGE_INDEX_PATTERNS renamed to _PACKAGE_MUTABLE_PATTERNS; all
built-in package-type index patterns (APKINDEX, repomd, manifests, etc.)
default to the remote's mutable_ttl (default 1 hour).

cache.file_ttl renamed to cache.immutable_ttl for consistency.
Adds github-archive remote to remotes.yaml as a worked example showing
tag archives as immutable and branch archives as mutable (1-day TTL).

docker-compose.yml: fix VERSION=dev → 2.2.2.dev0 (valid PEP 440),
add :z SELinux label to volume mounts.

docker-compose.yml

@@ -6,12 +6,12 @@ services:
       context: .
       dockerfile: Dockerfile
       args:
-        - VERSION=dev
+        - VERSION=2.2.2.dev0
     ports:
       - "8000:8000"
     volumes:
-      - ./remotes.yaml:/app/remotes.yaml:ro
-      - ./ca-bundle.pem:/app/ca-bundle.pem:ro
+      - ./remotes.yaml:/app/remotes.yaml:ro,z
+      - ./ca-bundle.pem:/app/ca-bundle.pem:ro,z
     environment:
       - CONFIG_PATH=/app/remotes.yaml
       - DBHOST=postgres