resolve the dialect in the repository factory instead of assuming sqlite (#1143)

createCurrentRepositoryContext() hardcoded `dialect: "sqlite"` while the
runtime already carried all three engines. That field is not decoration:
returning.ts reads it to decide whether it can ask for RETURNING, and
whether an upsert spells itself onConflictDoUpdate or onDuplicateKeyUpdate.
Reporting sqlite while connected to MySQL means the first upsert calls
onConflictDoUpdate on a mysql2 insert builder, which does not have it --
a TypeError, not a rejected query, as the note in returning.ts warned.

So MySQL never worked outside the tests, and Postgres worked only because
it also supports RETURNING and shares the conflict syntax.

Three things were supposed to catch this and none could. The repository
suite builds its own DatabaseContext in test-support.ts, verify-dialects.mjs
builds its own, and the CI matrix runs both against real Postgres and MySQL
containers -- all of them bypassing the one function the application calls.
Green on three engines, broken on two.

Resolve it from the environment, and test the factory itself rather than a
hand-built context: the default, each configured dialect, the write hook it
installs only for sqlite, and that an unsupported value throws rather than
falling back. Reverting the fix fails two of them.

Fixes Termix-SSH/Support#282

docs/database-backends.md

@@ -189,6 +189,19 @@ Tested against PostgreSQL 16 and MySQL 8. **MariaDB is not a substitute for
 MySQL when testing** — it accepts DDL that MySQL 8 rejects, which has hidden a
 real defect here more than once.
 
+### What neither of them covers
+
+Both harnesses build a `DatabaseContext` of their own, so neither runs
+`createCurrentRepositoryContext()` — the one the application actually uses.
+That gap hid a hardcoded `dialect: "sqlite"` in it: every engine reported
+itself as SQLite at runtime while all three test passes stayed green, which on
+MySQL meant `upsert` reached for `onConflictDoUpdate` and died with a
+TypeError on the first write.
+
+Anything the factory decides from the dialect needs its own test against the
+factory. Asserting it through a hand-built context proves nothing about what
+runs in production.
+
 ## Known limits
 
 - The desktop app always uses SQLite. It embeds its own backend and cannot ship