Add community text submissions, and make custom text usable for study

Submissions
- POST /api/texts proposes a passage; nothing reaches players until a
  moderator approves it. GET /api/texts serves the approved set, which the
  client merges on top of its bundled packs at startup.
- Validation the server enforces rather than trusts: category from a fixed
  list, 40 to 600 characters, no control characters (a newline makes a
  passage untypeable in a single-line input), attribution length, and a
  unique index on md5(lower(btrim(content))) so the same passage cannot be
  submitted twice under different whitespace or casing.
- Moderation is a flag on users. The queue and the review endpoint both
  refuse a non-moderator, and reviewing an already-reviewed submission is a
  404 rather than a silent second write.
- Submissions are rate limited per user: enough for a real contributor, not
  enough to fill the queue from a script.
- A Contribute screen carries the form, your own submissions with their
  status, and - for moderators only - the review queue.

This is the half of TypeRacer's model the packs could not reach by authoring:
their corpus is roughly twelve thousand passages, grown by submission.

Custom text as a study tool
- Imported documents are kept between visits, with how far through each one
  you are. Custom text lived only in memory, so importing a set of notes and
  reloading the page lost them - fine for pasting a paragraph to race,
  useless for working through a file over several sittings.
- Position is recorded when a segment is finished, not when the next is
  started, so closing the tab after a segment does not lose it.
- Markdown is chunked as markdown: fenced code blocks are kept whole and
  typed line by line, and the decoration - hashes, asterisks, backticks,
  link brackets, table pipes - is stripped so what you retype is the
  material rather than the punctuation around it.
- Everything stays on the device. Notes are not uploaded anywhere.

Fixed while doing it: a chunk could contain a newline, which cannot be typed
in a single-line input at all. Any paragraph with a line break inside it --
ordinary in notes and in wrapped prose - produced an unfinishable segment.
Whitespace inside a chunk is now flattened.

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