release: open days older than the first entry ever logged (#4)

## Fixed

- Days older than the first entry ever logged can now be opened. Picking a
  date from before anything was recorded landed on a dead end.

## Changed

- Releases are built by CI. Merging this pull request builds the image,
  tags it `vYYYYMMDD-N` and `latest`, pushes both, and creates the git
  tag. Nothing is built locally any more.
- `make image`, `push`, `release`, `seed`, `icons` and `vendor` are gone;
  the Makefile is down from 151 lines to 71. The two rare commands are
  written out in the README.
- Every push to `dev` now runs `make check` in CI.

## Deployment

No new or renamed environment variables, and `compose.yaml` is unchanged —
nothing to copy to the server this time. Pull the new image once CI reports
the build finished.

---------

Co-authored-by: Esa Kataja <[email protected]>
Reviewed-on: #4

.gitea/workflows/check.yaml

@@ -0,0 +1,22 @@
+name: check
+on:
+  push:
+    branches: [dev]
+
+# ponytail: only because Traefik still serves its default self-signed cert for
+# git.kessinen.com. Remove once the LE-DNS01-cloudflare runbook has been run.
+env:
+  GIT_SSL_NO_VERIFY: "true"
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - run: make check