docs: add database maturity badges and production scale note

[aparajon]

Why this matters

The README never says which databases SchemaBot supports or how mature each one is — PostgreSQL isn't mentioned at all, and the engines only appear mid-list in How It Works. A visitor evaluating SchemaBot has to read the docs or the code to answer "does it support my database, and can I bet production on it?". This puts that answer in the first screenful.

What it does

• Adds per-database maturity badges directly under the title: MySQL GA, Vitess GA, PostgreSQL early alpha.
• Expands the "We run what we ship" paragraph in Releases with the evidence behind those badges: SchemaBot runs the majority of Block's schema change traffic across a large fleet of MySQL and Vitess databases, including tables spanning many terabytes and keyspaces spanning 100s of shards.
• Links each engine from the Apply lifecycle line — Spirit, PlanetScale deploy requests, pg-sprite — adding PostgreSQL to the lifecycle description.

Rendered badge strip (under the title)

Rendered "We run what we ship" paragraph (Releases)

We run what we ship. Every tag is deployed to production at Block, where SchemaBot runs the majority of Block's schema change traffic across a large fleet of MySQL and Vitess databases (with PostgreSQL planned). We actively use SchemaBot to run schema changes against tables of many shapes and sizes, with some spanning many terabytes in MySQL and 100s of shards in Vitess. Our release cadence keeps the project continuously validated against real production workloads and gets fixes out fast. Because SchemaBot is pre-1.0, the release notes are the compatibility contract: give them a read before upgrading.

How it moves us toward the northstar

OSS adoption starts at the README: support and maturity have to be legible before anyone installs anything. This is the first step of the first-experience work — an honest support statement the upcoming schemabot init onboarding path can build on.

Opened by Claude (Fable 5).

[Copilot]

Pull request overview

Adds a “Database Support” section to the README to make SchemaBot’s supported engines and maturity level clear at a glance, and updates the lifecycle description to include PostgreSQL’s pg-sprite path.

Changes:

• Add a database support matrix (MySQL, Vitess, PostgreSQL) with maturity badges.
• Add a short maturity/positioning paragraph describing current production usage vs early-alpha PostgreSQL support.
• Update the “Apply” lifecycle bullet to include pg-sprite for PostgreSQL.

Suppressed comments (1)

README.md:63

• Now that PostgreSQL (pg-sprite) is included in the lifecycle, the next sentence reads as if "instant DDL" applies universally. "Instant DDL" is MySQL-specific terminology; scope this statement to MySQL to avoid misleading readers.

- **Apply** — execute DDL online using Spirit (MySQL), PlanetScale deploy requests (Vitess), or pg-sprite (PostgreSQL)
- **Progress** — track row copy progress, ETA, per-table/per-shard status
- **Control** — `stop` (pause), `start` (resume), `volume` (adjust speed), `cutover` (trigger table swap), `revert` (roll back)

Simple changes (e.g., adding a column) use instant DDL and complete in milliseconds. Operations that require a row copy (e.g., adding an index) run online without blocking reads or writes.

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