REST API OpenAPI spec (#612)

* OpenAPI spec in specs/api.  Checked lint with redoc.

specs/api/components/common/DocumentMetadata.yaml

@@ -0,0 +1,23 @@
+type: object
+description: Document metadata
+properties:
+  url:
+    type: string
+    description: Document URL
+    example: https://example.com/document.pdf
+  title:
+    type: string
+    description: Document title
+    example: Example Document
+  author:
+    type: string
+    description: Document author
+    example: John Doe
+  metadata:
+    type: object
+    description: Additional metadata
+    additionalProperties:
+      type: string
+    example:
+      department: Engineering
+      category: Technical

specs/api/components/common/ProcessingMetadata.yaml

@@ -0,0 +1,21 @@
+type: object
+description: Processing task metadata
+properties:
+  flow:
+    type: string
+    description: Flow ID
+    example: my-flow
+  collection:
+    type: string
+    description: Collection identifier
+    example: default
+  status:
+    type: string
+    description: Processing status
+    enum: [pending, processing, completed, failed]
+    example: processing
+  timestamp:
+    type: string
+    description: ISO timestamp
+    format: date-time
+    example: "2024-01-15T10:30:00Z"

specs/api/components/common/RdfValue.yaml

@@ -0,0 +1,14 @@
+type: object
+description: RDF value - can be entity/URI or literal
+required:
+  - v
+  - e
+properties:
+  v:
+    type: string
+    description: Value (URI or literal text)
+    example: https://example.com/entity1
+  e:
+    type: boolean
+    description: True if entity/URI, false if literal
+    example: true

specs/api/components/common/Triple.yaml

@@ -0,0 +1,16 @@
+type: object
+description: RDF triple (subject-predicate-object)
+required:
+  - s
+  - p
+  - o
+properties:
+  s:
+    $ref: './RdfValue.yaml'
+    description: Subject
+  p:
+    $ref: './RdfValue.yaml'
+    description: Predicate
+  o:
+    $ref: './RdfValue.yaml'
+    description: Object