Fix issues #002-#006 found by the LLM test harness

- #006 (high, data loss): make_room_for() no longer deletes unkept
  artifacts on a doomed request - checks whether eviction can possibly
  succeed before deleting anything. Also fixed a related latent bug found
  while testing: the function returned evicted Artifact objects after
  calling .delete() on them, so their .id was always None (Django clears
  pk on delete); now returns ids captured beforehand.
- #003: added server-side field-length validation (source/title/
  source_name/source_id/theme) on POST and PATCH, matching the model's
  own max_length limits, which were previously unenforced on the ingest
  path entirely.
- #004: ported daybook_post.py's content-shape validation
  (html_fragment/html_full/deck/markdown) to the server, closing the gap
  for any client that doesn't go through the helper script.
- #005: PATCH now rejects wrong-typed `tags` (previously silently wiped
  existing tags) and rejects immutable fields (content/source/kind/day)
  with 400 instead of silently no-op'ing them.
- #002: deleted the misleading skills/daybook-posting/.env.example (a
  zero-dependency script that never reads .env files) and clarified
  SKILL.md/docs/llm_posting_helper.md that these must be real
  process/shell env vars.

Adds tests/unit/ for storage-layer tests. 46 tests passing (was 25).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

issues/004-html-fragment-content-not-validated-server-side.md

@@ -0,0 +1,71 @@
+# 004 — `html_fragment`/`deck`/`html_full` content-shape rules exist only in the client helper, not the server
+
+**Status:** Fixed
+
+## Description
+
+`docs/api.md` and `skills/daybook-posting/SKILL.md` both describe a
+content-shape contract per text `kind`:
+
+- `html_fragment` — "Wrapped with a Daybook theme" (body-only HTML, no
+  `doctype`/`html`/`body`).
+- `html_full` — complete document, must contain `<html>`.
+- `deck` — top-level `<section>` blocks.
+
+`skills/daybook-posting/scripts/daybook_post.py`'s `validate_payload()`
+enforces all three of these client-side before ever sending the request.
+**`artifacts/views.py`'s `ArtifactListCreateView.post` / `create_text_artifact`
+enforce none of them** — the only server-side checks are "kind is one of
+the 4 text kinds" and "required fields present." Any client that talks to
+`POST /api/artifacts` directly (curl, a different language, a
+misconfigured agent) bypasses the contract entirely, and the server
+accepts and stores whatever is sent.
+
+Confirmed: posting `kind: "html_fragment"` with a full
+`<!doctype html><html><head>...</head><body>...</body></html>` as
+`content` returns `201`, and the stored page (`GET /a/{id}`) is:
+
+```html
+<!doctype html>
+<html>
+<head>
+  ...
+  <title>nested html fragment</title>
+  <link rel="stylesheet" href=".../report.css">
+</head>
+<body class="artifact">
+<!doctype html><html><head><title>Injected</title></head><body><h1>Nested doc</h1></body></html>
+</body>
+</html>
+```
+
+i.e. a doctype and a full `<html><head><body>` nested inside the theme
+wrapper's own `<html><head><body>` — invalid markup that browsers will
+silently "fix up" in unpredictable ways (duplicate/conflicting `<title>`,
+nested `<head>`, etc.), rather than the clean, themed fragment the kind
+is supposed to guarantee.
+
+This is the same class of gap as #003: validation the docs promise is
+implemented only in the optional client helper, not the server that's
+supposed to be the actual contract enforcement point. Related to (but
+distinct from) issue #003, which is about unenforced *length* limits
+rather than unenforced *content shape*.
+
+## Suggested Fix
+
+Port `daybook_post.py`'s `validate_payload()` checks (or equivalent) into
+`create_text_artifact`/`ArtifactListCreateView.post` server-side, returning
+`400` with the same messages the helper already uses. This also closes the
+gap for any future non-Python client.
+
+## Resolution
+
+Added `validate_content_shape(kind, content)` in `artifacts/views.py`,
+ported directly from `daybook_post.py`'s `validate_payload()` (same rules,
+same case-insensitive substring checks) so client and server agree: `deck`
+requires `<section`, `html_fragment` rejects `<html`/`<!doctype`/`<body`,
+`html_full` requires `<html`, and `markdown` rejects whitespace-only content
+(which the old required-field check missed, since a non-empty
+whitespace-only string is truthy). Called from `ArtifactListCreateView.post`
+after the existing field/source validation. Tests added covering each
+rejection case plus a well-formed `html_fragment` still succeeding.