feat: add free-form tags on transactions

Third of the four ideas picked from the Maybe/Sure feature audit. Tags are
orthogonal to categories (category = what kind of spend, tag = which
event/project) — many-to-many, created inline while typing in the
transaction form, filterable from the Transactions page.

Also adds tests/test_backup.py, the first test coverage the backup
export/import path has ever had, and fixes a MissingGreenlet bug it
surfaced in restore_backup's tag-wiring.

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

backend/app/schemas/backup.py

@@ -42,6 +42,13 @@ class CategoryBackup(BaseModel):
     parent_id: int | None = None
 
 
+class TagBackup(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    name: str
+
+
 class TransactionBackup(BaseModel):
     model_config = ConfigDict(from_attributes=True)
 
@@ -55,6 +62,11 @@ class TransactionBackup(BaseModel):
     merchant: str | None
     notes: str | None
     date: date_
+    # Defaulted so a backup exported before tags existed still imports
+    # cleanly under the same format version. Not a plain column — populated
+    # explicitly in build_backup() from the `tags` relationship, since
+    # from_attributes can't map a `tags` relationship to a `tag_ids` field.
+    tag_ids: list[int] = Field(default_factory=list)
 
 
 class AssetBackup(BaseModel):
@@ -155,6 +167,9 @@ class BackupPayload(BaseModel):
     app_version: str
     accounts: list[AccountBackup]
     categories: list[CategoryBackup]
+    # Defaulted so a backup exported before tags existed still imports
+    # cleanly under the same format version.
+    tags: list[TagBackup] = Field(default_factory=list)
     transactions: list[TransactionBackup]
     assets: list[AssetBackup]
     asset_valuations: list[AssetValuationBackup]

backend/app/schemas/tag.py

@@ -0,0 +1,12 @@
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class TagCreate(BaseModel):
+    name: str = Field(min_length=1, max_length=50)
+
+
+class TagRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    name: str

backend/app/schemas/transaction.py

@@ -7,6 +7,7 @@ from app.core.text import capitalize_first_letter
 from app.models.enums import TransactionType
 from app.schemas.account import AccountRead
 from app.schemas.category import CategoryRead
+from app.schemas.tag import TagRead
 
 
 class TransactionBase(BaseModel):
@@ -41,7 +42,7 @@ class TransactionBase(BaseModel):
 
 
 class TransactionCreate(TransactionBase):
-    pass
+    tag_ids: list[int] = Field(default_factory=list)
 
 
 class TransactionUpdate(BaseModel):
@@ -54,6 +55,8 @@ class TransactionUpdate(BaseModel):
     merchant: str | None = Field(default=None, max_length=150)
     notes: str | None = None
     date: date_ | None = None
+    # Omitted -> tags untouched; sent (even as []) -> replaces the full tag set.
+    tag_ids: list[int] | None = None
 
     @field_validator("description")
     @classmethod
@@ -67,6 +70,7 @@ class TransactionRead(TransactionBase):
     id: int
     account: AccountRead
     category: CategoryRead | None = None
+    tags: list[TagRead] = Field(default_factory=list)
 
 
 class TransactionPage(BaseModel):