Convert uploaded .xlsx files to csv artifacts server-side

Uploading a .xlsx file no longer becomes a generic downloadable binary -
it's read server-side with openpyxl and converted to a csv artifact,
rendering in the same interactive grid as a native CSV post. Detected by
content type or .xlsx extension (some clients send generic/incorrect
content types for multipart uploads). Only the active sheet converts;
formula cells use whatever cached result the spreadsheet software last
saved (openpyxl doesn't evaluate formulas itself). Corrupt/unreadable
files return 400.

Since the existing posting script already forwards any non-text file via
multipart with no special-casing, `daybook_post.py balance_sheet.xlsx`
already works with zero script changes - verified live.

Two real bugs found and fixed while building this, both now covered by
regression tests:
- {**a_querydict, ...} silently wrapped every value (including title) in
  a list - Python's ** spread on a dict subclass bypasses QueryDict's
  __getitem__ override and exposes its internal one-list-per-key storage
  directly. Fixed by building the delegated dict explicitly via .get().
- Pre-setting None for absent optional fields (e.g. source_prompt) defeated
  create_text_artifact's own data.get(field, default) calls, since .get()
  only falls through to its default when the key is *missing*, not when
  the value is None - caused a NOT NULL constraint failure. Fixed by only
  setting keys that were actually provided.
- csv.writer's default "\r\n" terminator was getting doubled by
  Path.write_text()'s own newline translation on Windows, producing a
  blank line between every row. Fixed with lineterminator="\n".

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

docs/api.md

@@ -226,14 +226,38 @@ Required fields:
 | `title` | Required. |
 | `file` | Required file part. |
 
-File kind is inferred from content type:
+File kind is inferred from content type (or file extension for `.xlsx`):
 
 | Content type | Artifact kind |
 |---|---|
 | `image/*` | `image` |
 | `application/pdf` | `pdf` |
+| `.xlsx` (by content type or extension) | `csv` — converted server-side, see below |
 | Anything else | `binary` |
 
+### `.xlsx` Upload → `csv` Kind
+
+Uploading a `.xlsx` file doesn't create a `binary` artifact — it's read
+server-side (`openpyxl`) and converted into a `csv` artifact, so it renders
+in the same interactive grid view as a native CSV post. Known limitations:
+
+- Only the workbook's **active sheet** is converted; other sheets in a
+  multi-sheet workbook are silently dropped.
+- Formula cells use the **cached result** the spreadsheet application last
+  calculated and saved, not a live re-evaluation — a workbook that's never
+  actually been opened/calculated in real spreadsheet software (e.g. one
+  built by a script that never triggered calculation) will show those cells
+  blank.
+- Legacy `.xls` (pre-2007 binary format) is **not** supported, only modern
+  `.xlsx`.
+- Rich spreadsheet formatting (bold, colors, merged cells, multi-level
+  headers) is not preserved — like any `csv` artifact, it renders as a
+  plain sortable/searchable table. If preserving that formatting matters
+  more than interactivity, post as `html_fragment` with a hand-built
+  `<table>` instead.
+
+A corrupt or unreadable `.xlsx` file returns `400`.
+
 PowerShell/curl example:
 
 ```powershell