* Speed up the startup prune's owned-prefix check

mark_contents_missing_outside_prefixes tested every live AssetContent row
against every owned prefix with Path.is_relative_to, which walks the path's
parents on each call: rows x prefixes x depth. At 9k rows and 150 prefixes
that was 9.5s (15.5s with deeper output paths).

path_prefix_matcher normalizes the prefixes once and checks each row with a
normcase'd, separator-bounded str.startswith over a tuple, keeping
is_relative_to's component bounds and platform case rules. The same case
now takes about 0.04s, flat in prefix count and depth.

* Move path_prefix_matcher next to the SQL containment predicate

path_utils needs the same check for per-file tagging, and scanner_changes
already imports path_utils, so the matcher moves to app.assets.helpers
beside sql_path_under_prefix.

* Speed up asset tag derivation during scans

get_backend_system_tags_from_path checked every scanned file against every
model base with Path.is_relative_to, which dominated the fast scan: 37s of
build_asset_specs for 100k output files. Using path_prefix_matcher, the
separator-bounded string check the startup prune already uses, brings that
to about 7s.

* Keep path_prefix_matcher's parity for paths starting with two slashes

abspath keeps exactly two leading slashes, and pathlib treats that '//' as an
anchor of its own, so Path('//server/f').is_relative_to('/') is False, but
the string check accepted it. Prefixes are now split once by whether their
anchor is '//', and a candidate is only checked against prefixes with the
same anchor. The per-row check is still a single str.startswith.

* Build tag-derivation prefix matchers once per folder config

get_backend_system_tags_from_path built a path_prefix_matcher for input,
output, temp and every model category on each call, so most of its cost was
normalizing the same prefixes again for every scanned file. cached_prefix_matcher
memoizes construction on the raw prefix tuple, which the callers pass as
absolute folder paths; a folder-config change is a new key.

100k files, 58 model bases: tag derivation 81.8 -> 27.7 us/file, and
build_asset_specs CPU 11.55 -> 6.40s.

* State cached_prefix_matcher's absolute-prefix precondition

The docstring presented absolute prefixes as a property of the callers. It
is a precondition the caller must meet: folder_paths stores what it is given,
and a relative prefix is resolved once, on first use, and then frozen.

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Co-authored-by: guill <jacob.e.segal@gmail.com>