When a file is concurrently deleted while addRecipient runs, the FK
constraint on recipients.sender_id raises ForeignKeyViolation. Previously
this propagated as INTERNAL; now it returns AUTH (file not found).

expireServerFiles unconditionally subtracted file_size from usedStorage
for every expired file, including files that were never uploaded (no
file_path). Since reserve only increments usedStorage during upload,
expiring never-uploaded files caused usedStorage to drift negative.

setFilePath result was discarded with void. If it failed (file deleted
concurrently, or double-upload where file_path IS NULL guard rejected
the second write), the server still reported FROk, incremented stats,
and left usedStorage permanently inflated. Now the error is checked:
on failure, reserved storage is released and AUTH is returned.

The status field (e.g. "blocked,reason=spam,notice={...}") is quoted in
CSV for COPY protocol, but embedded double quotes from BlockingInfo
notice (JSON) were not escaped. This could break CSV parsing during
import. Now double quotes are escaped as "" per CSV spec.

In Postgres mode, getFile returns a snapshot TVar for fileStatus. If a
file is blocked between getFile and setFilePath, the stale status check
passes but the upload should be rejected. Added status = 'active' to
the UPDATE WHERE clause so blocked files cannot receive uploads.

Prevents negative or zero file_size values at the database level.
Without this, corrupted data from import or direct DB access could
cause incorrect storage accounting (getUsedStorage sums file_size,
and expiredFiles casts to Word32 which wraps negative values).

importFileStore now checks if the target database already contains
files and aborts with an error. Previously, importing into a non-empty
database would fail mid-COPY on duplicate primary keys, leaving the
database in a partially imported state.

When setFilePath fails (file deleted or blocked concurrently, or
duplicate upload), the uploaded file was left orphaned on disk with
no DB record pointing to it. Now the file is removed on failure,
matching the cleanup in the receiveChunk error path.

The store action result (deleteFile/blockFile) was discarded with void.
If the DB row was already deleted by a concurrent operation, the
function still decremented usedStorage, causing drift. Now the error
propagates via ExceptT, skipping the usedStorage adjustment.

deleteFile result was discarded with void. If a concurrent delete
already removed the file, deleteFile returned AUTH but usedStorage
was still decremented — causing double-decrement drift. Now the
usedStorage adjustment and filesExpired stat only run on success.

- Move STMFileStore and its FileStoreClass instance from Store/STM.hs
  back into Store.hs — the separate file was unnecessary indirection
  for the always-present default implementation.

- Parameterize xftpFileTests over store backend using HSpec SpecWith
  pattern (following SMP's serverTests approach). The same 11 tests
  now run against both memory and PostgreSQL backends via a bracket
  parameter, eliminating all *Pg test duplicates.

- Extract shared run* functions (runTestFileChunkDeliveryAddRecipients,
  runTestWrongChunkSize, runTestFileChunkExpiration, runTestFileStorageQuota)
  from inlined test bodies.

- Remove internal helpers from Postgres.hs export list (withDB, withDB',
  handleDuplicate, assertUpdated, withLog are not imported externally)
- Replace local isNothing_ with Data.Maybe.isNothing in Env.hs
- Consolidate duplicate/unused imports in XFTPStoreTests.hs
- Add file_path IS NULL and status guards to STM setFilePath, matching
  the Postgres implementation semantics