[datlechin]

Summary

• Show ER Diagram, in a table's or view's sidebar menu and on each schema's heading, opens the schema in a tab of its own. A second Show brings that tab forward. A key-value engine's keys offer no diagram.
• Tables are cards of their columns with PK and FK badges; foreign keys are lines between the rows they join, in crow's foot notation (nullable and unique keys drawn as such). Keys to other schemas are not drawn.
• The tables are read in the background, two at a time on one lease each (columns, foreign keys, indexes), with progress and Stop. A refresh keeps the diagram up, shows its progress in the bottom bar and turns Refresh into Stop. A failed refresh keeps the diagram under a Retry banner; a schema change shows a Refresh banner.
• Automatic layout: referenced tables left of the tables that hold their keys, barycenter ordering, long columns wrapped, lone tables in a grid.
• Pan (drag empty space, middle drag, scroll), zoom (buttons, Ctrl+plus/minus/0, f for Best Fit, Ctrl+scroll, pinch), drag tables, Reset Layout. Moved tables keep their place through a refresh, a reopen (Ctrl+Shift+T) and a restart.
• Click selects a table and highlights its keys at both ends; double click or Enter opens it in a table tab; the menu (right click, Menu, Shift+F10) offers Open Table and Show Structure.
• Export as PNG (2x, capped) or SVG, drawn through the same render nodes as the screen. The file is written through the workspace's export ledger: Stop in the operations popover, the close prompt, the logout inhibitor and an "Exported" toast with Show in Files.
• Keyboard and screen reader: the canvas is a list and each table a focusable list item with its name, position and a description of its columns and the tables it joins. Tab and the arrow keys move between tables, Alt+arrows move one, the focus selects it. Hidden controls hand the focus on, and a drawn diagram is announced.
• Colours come from the theme (libadwaita colour variables read through hidden probes, the accent colour), so light, dark and high contrast follow the desktop.
• Enables cairo-rs's png and svg features; no new crate.
• Workspace file version 2 (Diagram tab records with moved places); version 1 reads unchanged through an identity step, with a workspace-v2.json golden fixture that holds every tab kind.
• New "ER Diagram" section in the shortcuts dialog; ROADMAP and ARCHITECTURE updated.

Test plan

• Unit: model (keys, other schemas, uniqueness, nullability), layout (no overlap, direction, cycles, 500 tables), scene routing and hits, zoom, palette, export PNG and SVG, places budget, records round trip.
• Widget: every table named and focusable, focus selects and Enter opens, keyboard moves kept through a refresh, zoom, Stop, a failed refresh keeps the diagram, the empty schema, 500 tables in bounded time, the bar within the 360 px window floor while refreshing, no diagram offered for a key-value engine's keys; workspace: one tab per schema, sidebar menu and heading, Open Table and Show Structure, restore and reopen with places, schema change banner, theme colours.
• Updated on purpose: the_new_table_button_follows_the_dialect finds the New Table button by its action, since the schema heading now has two buttons; unsupported_version_blocks_key seeds version 3, since 2 is now current.
• Real engines (requires docker): a 50-table schema with 54 keys on PostgreSQL, MySQL, MariaDB and SQL Server; SQLite in the unit suite.

Screenshots

Fixes #3670