refactor(forge): extract ForgeBackend class and registry

Problem: adding a new forge required touching 5 lookup tables
(`FORGE_HOSTS`, `FORGE_CLI`, `FORGE_AUTH_CMD`, `SHORTHAND_PREFIX`,
`_warned_forges`) and every branching site in `_api_args`,
`fetch_metadata`, and `parse_ref`.

Solution: introduce a `ForgeBackend` class with `parse_url`,
`api_args`, and `parse_state` methods, plus a `register()` /
`backends()` registry. New forges (Gitea, Forgejo) are a single
`register()` call via the `gitea_backend()` convenience constructor.

spec/forge_spec.lua

@@ -316,6 +316,81 @@ describe('forge parse.body integration', function()
   end)
 end)
 
+describe('forge registry', function()
+  it('backends() returns all registered backends', function()
+    local backends = forge.backends()
+    assert.is_true(#backends >= 3)
+    local names = {}
+    for _, b in ipairs(backends) do
+      names[b.name] = true
+    end
+    assert.is_true(names['github'])
+    assert.is_true(names['gitlab'])
+    assert.is_true(names['codeberg'])
+  end)
+
+  it('register() with custom backend resolves URLs', function()
+    local custom = forge.gitea_backend({
+      name = 'mygitea',
+      shorthand = 'mg',
+      default_host = 'gitea.example.com',
+    })
+    forge.register(custom)
+
+    local ref = forge.parse_ref('https://gitea.example.com/alice/proj/issues/7')
+    assert.is_not_nil(ref)
+    assert.equals('mygitea', ref.forge)
+    assert.equals('alice', ref.owner)
+    assert.equals('proj', ref.repo)
+    assert.equals('issue', ref.type)
+    assert.equals(7, ref.number)
+  end)
+
+  it('register() with custom shorthand resolves', function()
+    local ref = forge._parse_shorthand('mg:alice/proj#7')
+    assert.is_not_nil(ref)
+    assert.equals('mygitea', ref.forge)
+    assert.equals('alice', ref.owner)
+    assert.equals('proj', ref.repo)
+    assert.equals(7, ref.number)
+  end)
+
+  it('_api_args dispatches to custom backend', function()
+    local args = forge._api_args({
+      forge = 'mygitea',
+      owner = 'alice',
+      repo = 'proj',
+      type = 'issue',
+      number = 7,
+      url = '',
+    })
+    assert.same({ 'tea', 'api', '/repos/alice/proj/issues/7' }, args)
+  end)
+
+  it('gitea_backend() creates a working backend', function()
+    local b = forge.gitea_backend({
+      name = 'forgejo',
+      shorthand = 'fj',
+      default_host = 'forgejo.example.com',
+      cli = 'forgejo-cli',
+      auth_cmd = 'forgejo-cli login',
+    })
+    assert.equals('forgejo', b.name)
+    assert.equals('fj', b.shorthand)
+    assert.equals('forgejo-cli', b.cli)
+
+    local ref = b:parse_url('https://forgejo.example.com/bob/repo/pulls/3')
+    assert.is_nil(ref)
+
+    forge.register(b)
+    ref = b:parse_url('https://forgejo.example.com/bob/repo/pulls/3')
+    assert.is_not_nil(ref)
+    assert.equals('forgejo', ref.forge)
+    assert.equals('pull_request', ref.type)
+    assert.equals(3, ref.number)
+  end)
+end)
+
 describe('forge diff integration', function()
   local store = require('pending.store')
   local diff = require('pending.diff')