Add plain postgres models, and stop doubling % in constraint literals

[davegaeddert]

What

plain postgres models describes the data model as the registry sees it, with no database connection: every field, every relation with its target and on_delete rule, the relations pointing back at each model with any declared reverse accessor, and constraints and indexes with their conditions compiled to SQL. --json carries the same facts as data; a target narrows it to a model (class name, label or table) or a package; --all adds the installed packages' models. The same facts are available from plain.postgres.describe.

unique and 1:1 mean effective uniqueness: an unconditional, single-field unique constraint. A partial unique is listed with its WHERE clause and never promoted, so "one open thread per bot" can't be mistaken for "one thread per bot".

quote_value no longer doubles % in string literals. The DDL it feeds runs with no parameters, so the doubled form was stored as written: CheckConstraint(check=~Q(name="100%")) was created as '100%%' and let the one value it names through.

Upgrade path. Existing databases hold the doubled form. Convergence now recognizes a stored definition that differs from the model only by that doubling and replaces the constraint under its own name without staging: a check in one DROP CONSTRAINT, ADD CONSTRAINT ... NOT VALID statement then validated; a unique rebuilt concurrently under a temporary name and swapped in inside one transaction. The table is never without the constraint. A failed replacement blocks sync. Plain indexes already rebuilt automatically.

Known limit: a model edited from a 100%% literal to 100% produces the same stored text and is replaced in place rather than staged under a new name. The validation still has to pass.

Also: Options.unique_field_names, PackageConfig.is_app replacing three prefix checks, a public reversed_field on reverse descriptors, and plain postgres schema sharing the same target matching (it now accepts a package label too).

Verified

• plain-postgres suite: 1824 passed. plain core: 900 passed. Type checks clean.
• Run against spur/cloud, HomeQuarter, Ranger, GKB, Plain public and the example app with the branch's plain-postgres, including with an unreachable database URL.
• Three staged convergence tests reproduce the old DDL through the old quoting and verify check constraints and partial uniques are replaced, while a genuinely changed check still blocks.

[pullapprove5]

PASS: 1 review scope passed

	Scope	Progress
✅	code	1/1

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Next steps:

• @davegaeddert: Merge or close