Cross-ruleset compendium browser: dedupe SRD entries with source selector

[barrulus]

Background

The SRD database holds the same entity (e.g. "fireball", "fighter", "longsword") once per loaded ruleset, by design. Currently three sources are loaded:

Source	Spells
`srd-2014`	319
`srd-2024`	338
`a5e-ag` (Level Up Advanced 5e)	371

Same pattern applies to species, classes, backgrounds, feats, items, weapons, and armor — see `database/srd-schema-multi-source.sql`, which deliberately replaced `UNIQUE(key)` with `UNIQUE(key, document_source)`.

Current state (after #26 fix)

The list endpoints in `server/routes/srd.routes.js` (`/spells`, `/species`, `/classes`, `/backgrounds`, `/feats`, `/items`) now default to `?source=srd-2024` when no source is given, and support `?source=all` to opt into a cross-ruleset view.

This fixes the immediate duplication symptom in the compendium UI but is not the right long-term answer:

• A player whose character is built against `srd-2014` or `a5e-ag` sees the wrong spell list.
• There is no way in the UI to compare versions of the same spell side-by-side (useful for DMs reconciling rulesets).
• Cross-ruleset browsing requires manually appending `?source=all`, which then re-introduces the duplication.

Proposal

Build a proper cross-ruleset compendium browser:

1. Backend: add a `/api/srd/spells/grouped` (and equivalents) that returns one row per `key`, with an array of available sources and per-source field deltas. Order sources by preference (`srd-2024 → srd-2014 → a5e-ag → other`), and surface a default "primary" version.

2. Frontend: render one card per spell with:

   • A source selector chip (`2024` / `2014` / `A5e`) showing which rulesets define this spell
   • The currently-selected version's text
   • A "compare versions" affordance if multiple sources differ

3. Character context: when the user is viewing the compendium from a character sheet, default the source selector to that character's `srd_document_source`.

4. Search/filter: ensure filters (level, school, class, ritual, etc.) operate on the union across sources, not on a single source.

Out of scope (for this issue)

• Loading additional rulesets beyond what's already imported.
• Letting players "pin" a preferred ruleset per campaign — that's a separate config concern.

Related

• Schema drift between schema.sql and migrations causing recurring papercuts #26 — schema drift on `session_player_actions` (different concern, same family of "multi-source design without UI awareness" papercuts)
• The one-line default-filter fix landed in `server/routes/srd.routes.js` for all six list endpoints, which is what unblocks the immediate compendium duplication.