web: render memory bodies as markdown, and resolve their references

A memory's value is markdown and always was: `bd remember` stores what was
typed, and what is typed is the same prose the memory files carry — bold
leaders, code spans, fenced recipes, and [[slug]] references to related
memories. The view printed it as pre-wrapped paragraphs, which shows the
source rather than the document and leaves every reference a dead end.

The renderer is web/markdown.go, one shared goldmark with three departures
from stock:

  - [[slug]] is an inline parser rather than a text rewrite, so a reference
    written inside a code span stays text by construction.
  - Raw HTML is escaped and shown, not dropped. The safe default omits it,
    and these memories are full of <placeholder> spellings CommonMark reads
    as tags; omitting one rewrites SRHT_<NAME>_VER to SRHT__VER silently,
    which is worse than showing markup.
  - An image renders as a link. An <img> at another host is a request that
    host makes on behalf of whoever opened the page.

References resolve across databases, because that is where they point: the
mirroring workflow files a memory by its type, so a related memory is as
likely to be in another tracker as in this one. The index is the one the
issue links already use — issue_prefix and the kv.memory.* keys are rows of
the same config table, so one read per database now answers both questions
and one cached projection carries both. A slug two trackers hold resolves to
the first listed, the page's own database, unlike a prefix two trackers claim
which resolves to neither: the same slug in two trackers is normally one
memory that was re-filed and left a copy behind.

A slug nobody wrote and a slug held by a database the caller may not browse
are one rendering — a muted, inert marker. A link, a distinct class or a
different tooltip would each publish the existence of a private database.

Memory.Paragraphs goes with the paragraphs it existed for.