db: list every database a viewer may see

fix(db): map repository_path_key to ErrNameTaken

CreateRepo only recognized uq_repo_owner_id_name as a name collision, but
path is derived from (owner, name) by RepoDiskPath, so a duplicate always
violates both indexes -- and repository_path_key, declared inline on the
column, has the lower OID and is the one Postgres reports. ErrNameTaken was
therefore unreachable in practice and every duplicate surfaced as a raw
23505, breaking all three callers that branch on it:

  - /internal/repos returned 500 "create database" instead of an idempotent
    200, so git.sr.ht's post-update hook printed "companion provisioning
    failed (500)" on every push to a repo whose companion already existed
  - the web create form returned 500 instead of 409 "You already have a
    database with that name."
  - a lost remotesapi auto-create race failed with codes.Unavailable
    instead of adopting the winner's row

TestCreateRepoDuplicateName missed it by re-creating under a different
path, which only the name index catches; it is now table-driven over both.

rename module to sourcecraft.dev/bigbes/sr-ht-dolt; depend on sourcecraft sr-ht-core

db: postgres layer for repos, ACLs, dolt keys

Store wraps a Querier (*sql.DB/*sql.Tx/*sql.Conn); context-first methods, FromContext for core-go middleware, WithTx for the create-repo transaction. Repo CRUD + listing-visibility rules, effective-access/ACL upsert, dolt_key CRUD with typed ErrNotFound/ErrNameTaken/ErrKeyExists. Tests gated on DOLTSRHT_TEST_PG: per-run scratch schema + schema.sql, CRUD/visibility/effective-access cases; skip when unset.