feat(projects): three tables, schema 20 -> 21

Phase 2 of docs/project-feature.md. Still nothing that writes a row: this is the
shape, the keys and the queries each screen will want.

A project's contents are real foreign keys into this room's own rows, with
CASCADE, like every reference in this family. That is the one-room scope
decision made load-bearing rather than documented: a membership naming a version
this device does not hold fails at insert, loudly, instead of landing as a row
pointing at nothing that every later read has to be defensive about. Projects
that span rooms take those keys away again -- Phase 9, following
`parentChatRoomId`, which is deliberately not a foreign key because a dangling
value is the normal state there -- and that is a migration rather than something
to carry unused until then. `MantraProjectDaoJvmTest` asserts the refusal, which
is also what says Room has foreign keys switched on at all.

The owners are a comma-joined sorted string with an `ownerPublicKeySet()`
accessor beside it, which is what `DkgSession.participantPublicKeys` already
does with a set of keys, sort included and for the same reason. A fourth table
would buy querying by owner, which no screen does. A set stored as text is a
shape that rots, so the accessor is tested against spacing, empty entries and
the single-entry case rather than the happy path.

`fromProjectEvent` refuses an event with no name, no creator or no owners rather
than filling the gap from local knowledge -- which is exactly the failure the
tags exist to prevent. The empty-owner refusal does double duty: an owner list
that comes back empty is what `ROLE_OWNER` missing from `KNOWN_ROLES` looks like
from here, so the guard against a malformed event is also the alarm for that
bug.

No `toProjectEvent()`. Those eight rebuilds serve rooms whose work predates
`GroupSignedEvent` at v13; projects arrive at v21, so every project this app
will ever hold is on file as a signed event. Adding three more would be adding
to a mechanism with a removal checklist already written for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/press/mantra/compose/database/MantraDatabase.kt

@@ -29,6 +29,9 @@ import press.mantra.compose.database.dao.MantraChapterDao
 import press.mantra.compose.database.dao.MantraChunkDao
 import press.mantra.compose.database.dao.MantraDao
 import press.mantra.compose.database.dao.MantraDialectDao
+import press.mantra.compose.database.dao.MantraProjectArtifactVersionDao
+import press.mantra.compose.database.dao.MantraProjectDao
+import press.mantra.compose.database.dao.MantraProjectTranslationArtifactVersionDao
 import press.mantra.compose.database.dao.MantraTranslationArtifactVersionDao
 import press.mantra.compose.database.dao.MantraTranslationArtifactVersionContributorDao
 import press.mantra.compose.database.dao.MantraTranslationChapterDao
@@ -81,6 +84,9 @@ import press.mantra.compose.database.model.MantraArtifactVersion
 import press.mantra.compose.database.model.MantraChapter
 import press.mantra.compose.database.model.MantraChunk
 import press.mantra.compose.database.model.MantraDialect
+import press.mantra.compose.database.model.MantraProject
+import press.mantra.compose.database.model.MantraProjectArtifactVersion
+import press.mantra.compose.database.model.MantraProjectTranslationArtifactVersion
 import press.mantra.compose.database.model.MantraTranslation
 import press.mantra.compose.database.model.MantraTranslationArtifactVersion
 import press.mantra.compose.database.model.MantraTranslationArtifactVersionContributor
@@ -146,6 +152,9 @@ val GENESIS_AT = Instant.fromEpochMilliseconds(1231006505000L)
         MantraChapter::class,
         MantraChunk::class,
         MantraDialect::class,
+        MantraProject::class,
+        MantraProjectArtifactVersion::class,
+        MantraProjectTranslationArtifactVersion::class,
         MantraTranslation::class,
         MantraTranslationArtifactVersion::class,
         MantraTranslationArtifactVersionContributor::class,
@@ -177,7 +186,7 @@ val GENESIS_AT = Instant.fromEpochMilliseconds(1231006505000L)
         UnsignedNostrEvent::class,
         Zap::class
     ],
-    version = 20,
+    version = 21,
     autoMigrations = [
         // v2 only adds the DkgSession/DkgParticipantMessage tables, so Room can
         // generate the migration itself — nothing existing changes shape.
@@ -321,6 +330,22 @@ val GENESIS_AT = Instant.fromEpochMilliseconds(1231006505000L)
         // which is what they were, and any identity may drain them. The broadcast
         // queue deliberately gets no owner: a signed event is anyone's to carry.
         AutoMigration(from = 19, to = 20),
+        // v21 adds the three project tables -- a project, and one row per source
+        // version or translation that is part of it. Three new tables and
+        // nothing existing changes shape, so Room generates this itself.
+        //
+        // A project's contents are foreign keys into this room's own rows, with
+        // CASCADE, like every other reference in this family. That is the
+        // one-room scope decision made load-bearing: a membership naming a
+        // version this device does not hold fails at insert rather than landing
+        // as a row pointing at nothing. Projects that span rooms take those keys
+        // away again -- see Phase 9 of docs/project-feature.md -- and that is a
+        // migration rather than something to carry unused until then.
+        //
+        // The owners are a comma-joined sorted string on the project row rather
+        // than a fourth table, which is what DkgSession.participantPublicKeys
+        // already does with a set of keys and for the same reason.
+        AutoMigration(from = 20, to = 21),
     ]
 )
 @ColumnTypeConverters(MantraConverters::class)
@@ -368,6 +393,12 @@ abstract class MantraDatabase: RoomDatabase() {
 
     abstract fun mantraDialectDao(): MantraDialectDao
 
+    abstract fun mantraProjectDao(): MantraProjectDao
+
+    abstract fun mantraProjectArtifactVersionDao(): MantraProjectArtifactVersionDao
+
+    abstract fun mantraProjectTranslationArtifactVersionDao(): MantraProjectTranslationArtifactVersionDao
+
     abstract fun mantraTranslationArtifactVersionDao(): MantraTranslationArtifactVersionDao
 
     abstract fun mantraTranslationArtifactVersionContributorDao(): MantraTranslationArtifactVersionContributorDao