Let a baseline meet a table that is already there

Reported: `flask plugin upgrade-all` failed the tools plugin with "table
securesendsecrets already exists".

Every post-cutover baseline emitted a plain op.create_table. That is correct
only while a table's existence and its plugin's Alembic stamp can never
disagree, and they can. Anything that builds tables from the models -
db.create_all(), which `flask db-utils create-all` calls - creates them and
stamps nothing. A run that creates a table and then fails before its stamp
commits leaves the same split, and re-running is the obvious thing to try. In
both cases upgrade-all meets existing tables with an unstamped chain and the
create is fatal.

Nothing caught it because the migration test DROPS the post-cutover tables
db.create_all() built before running upgrade-all. That arranges the one state
where an unguarded create works, and never the state the report came from. The
new test is the same sequence with the drop removed for the plugin under test.

create_table_if_missing and create_index_if_missing are in alembic_template.py
next to create_plugin_tables, which had the idempotency rule right and turns out
to be called by nobody - its docstring claimed every 0001 used it. Fixed the
docstring rather than rewiring the baselines onto it, because the reason they
hand-write their DDL still stands: the helper's per-plugin MetaData cannot
resolve a foreign key to a core table at CreateTable-compile time.

INDEXES ARE MATCHED ON COLUMNS, NOT NAME, and that distinction matters. The two
creators disagree about names - db.create_all() takes SQLAlchemy's default
convention (ix_securesendsecrets_token) and a baseline spells it the way this
repo's naming rules require (ixsecuresendsecretstoken). A name-keyed guard would
have called the index missing and added a second one over the same column, which
MySQL permits with a warning, so every affected site would have quietly grown
duplicates. The test asserts no table ends up with two indexes over the same
columns.

Applied to all nine table-creating plugin migrations, not just the tools
baseline that was reported: the four later migrations that create tables have
the same exposure, and geenforce, measuringtools, printedparts and backups are
the same defect waiting for a different site.

WHAT THIS DOES NOT FIX. It makes a baseline idempotent, not a whole chain. A
longer chain can still meet an ALTER whose column is already present -
printedparts adding gagelabtag to a table create_all built at head is the case
that showed up while testing. Guarding every op one at a time is the wrong
answer to that; a database built from the models is already AT head and its
chains want stamping rather than replaying, and there is no `flask plugin stamp`
today. Left alone deliberately rather than half-built, and the new test is
scoped to single-baseline plugins so it does not pretend otherwise.

CHANGELOG.md

@@ -105,6 +105,20 @@ ADR-007 and ADR-002.
   the wrong code. The page index now travels with the page.
 - The back label's dotted preview outline outspecified the print reset and drew
   a dashed rule down the edge of every second card.
+- **`flask plugin upgrade-all` failed a plugin whose tables already existed**
+  ("table securesendsecrets already exists" on the tools plugin). Every
+  post-cutover baseline emitted a plain `op.create_table`, which is correct only
+  while a table's existence and its plugin's Alembic stamp cannot disagree.
+  They can: anything that builds tables from the models stamps nothing, and a
+  run that creates a table then fails before its stamp commits leaves the same
+  split, where re-running is the obvious response. Creates and indexes across
+  all nine table-creating plugin migrations now skip what is already there.
+  Indexes are matched on their COLUMNS rather than their name, because
+  `db.create_all()` names them by SQLAlchemy's convention and a migration by
+  this repo's, so a name-keyed check would silently add a duplicate index
+  instead of skipping. Note the remaining limit: this makes a baseline
+  idempotent, not a whole chain. A longer chain can still meet an `ALTER` whose
+  column is already present, which no guard on `create_table` addresses.
 - **Saving a machine that has a controlling PC answered "this relationship
   already exists" and refused the save.** Reported against a map position
   change, but the map was incidental: the form re-POSTed the controlling-PC