repo: add LICENSE, THIRD_PARTY.md, .dockerignore, hadolint lint, IDEAS backlog

Repo/CI hygiene batch (none base-affecting; image contents unchanged):

- LICENSE: actual MIT file (repo previously declared MIT only in prose).
- THIRD_PARTY.md: notes bundled software + licenses (pi/pi-fork/pi-obsmem/
  pi-studio MIT, gosu Apache-2.0, Debian packages under their own terms).
- .dockerignore: trims build context to what the Dockerfiles COPY (rootfs/ +
  entrypoint*.sh); keeps .git/docs/scripts/compose out. Verified it excludes
  none of the required COPY sources.
- lint.yml: new hadolint job (pinned v2.14.0) lints both Dockerfiles;
  .hadolint.yaml grandfathers deliberate choices (DL3008/DL3016/DL4006/DL3003/
  SC2086, mirroring the shellcheck excludes), fails on anything new at warning+.
  Verified hadolint exit 0 and the repo shell-guard passes with the new job.
- IDEAS.md: parks deferred follow-ups (SHA-pin actions, trivy, buildx SBOM/
  provenance, Makefile, renovate).
- README/DOCKER_HUB License sections now link LICENSE + THIRD_PARTY.md.

No tag.

.gitea/workflows/lint.yml

@@ -1,4 +1,4 @@
-name: Lint workflows
+name: Lint
 
 # Durable guard against CI-workflow bugs — most importantly the recurring
 # "bash-only syntax under the default `sh`/dash shell" footgun that broke
@@ -67,3 +67,27 @@ jobs:
         # ("no project was found"). Globbing the workflow files is the
         # supported way to lint a non-GitHub layout.
         run: actionlint -color .gitea/workflows/*.yml
+
+  hadolint:
+    # Lint the two Dockerfiles that ARE the project (the shell/actions linting
+    # above never looked at them). Config — ignored rules + failure threshold
+    # — lives in .hadolint.yaml, which hadolint reads automatically, so a local
+    # `hadolint Dockerfile.base` reproduces CI exactly.
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install hadolint (pinned)
+        env:
+          HADOLINT_VERSION: 2.14.0
+        run: |
+          curl -fsSL \
+            "https://github.com/hadolint/hadolint/releases/download/v${HADOLINT_VERSION}/hadolint-Linux-x86_64" \
+            -o /usr/local/bin/hadolint
+          chmod +x /usr/local/bin/hadolint
+          hadolint --version
+
+      - name: Run hadolint
+        run: hadolint Dockerfile.base Dockerfile.variant