Give every blog its own Postgres database

A blog is now a database of its own (blog_<sub>) on the same server: one
pg_dump is a complete backup of a blog, one psql restores it, and nothing a
blog's queries do can reach another blog's rows. The control database
(DATABASE_URL) keeps only users and the blog registry (id, owner, subdomain,
db_name); title, tagline and theme move into a one-row settings table next
to the content so the dump really is everything.

db.Cluster holds the control pool plus small, lazily opened per-blog pools.
store.Store (control) hands out a store.BlogStore per blog; every blog_id
parameter and column is gone, the database is the scope. Handlers reach it
through blogStore(r), which resolveBlog puts in the context next to the blog.

Existing data is moved in place by control migration 00006, a Go migration
that runs inside the control transaction: it creates and migrates each blog
database, copies the rows preserving ids, and marks the registry; 00007 then
drops the old tables. Either every blog is moved or the control database is
untouched.

/media/{id} now serves the host's blog only, so dashboard previews on the
root domain use /b/{sub}/media/{id}. Subdomains are capped at 58 chars so
"blog_" + name fits a Postgres identifier. Deleting a user drops their
database. Store.Open resets a blog's pool and retries once so a database
restored under a running app (dropdb --force, createdb, psql) just works.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sd8UPWrvyYCLj97JexNw3A

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