REST API OpenAPI spec (#612)

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

specs/api/components/schemas/embeddings-query/DocumentEmbeddingsQueryRequest.yaml

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+type: object
+description: |
+  Document embeddings query request - find similar documents by vector similarity.
+required:
+  - vectors
+properties:
+  vectors:
+    type: array
+    description: Query embedding vector
+    items:
+      type: number
+    example: [0.023, -0.142, 0.089, 0.234, -0.067, 0.156]
+  limit:
+    type: integer
+    description: Maximum number of document chunks to return
+    default: 10
+    minimum: 1
+    maximum: 1000
+    example: 20
+  user:
+    type: string
+    description: User identifier
+    default: trustgraph
+    example: alice
+  collection:
+    type: string
+    description: Collection to search
+    default: default
+    example: research

specs/api/components/schemas/embeddings-query/DocumentEmbeddingsQueryResponse.yaml

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+type: object
+description: Document embeddings query response
+properties:
+  chunks:
+    type: array
+    description: Similar document chunks (text strings)
+    items:
+      type: string
+    example:
+      - "Quantum computing uses quantum mechanics principles for computation..."
+      - "Neural networks are computing systems inspired by biological neurons..."
+      - "Machine learning algorithms learn patterns from data..."

specs/api/components/schemas/embeddings-query/GraphEmbeddingsQueryRequest.yaml

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+type: object
+description: |
+  Graph embeddings query request - find similar entities by vector similarity.
+required:
+  - vectors
+properties:
+  vectors:
+    type: array
+    description: Query embedding vector
+    items:
+      type: number
+    example: [0.023, -0.142, 0.089, 0.234, -0.067, 0.156]
+  limit:
+    type: integer
+    description: Maximum number of entities to return
+    default: 10
+    minimum: 1
+    maximum: 1000
+    example: 20
+  user:
+    type: string
+    description: User identifier
+    default: trustgraph
+    example: alice
+  collection:
+    type: string
+    description: Collection to search
+    default: default
+    example: research

specs/api/components/schemas/embeddings-query/GraphEmbeddingsQueryResponse.yaml

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+type: object
+description: Graph embeddings query response
+properties:
+  entities:
+    type: array
+    description: Similar entities (RDF values)
+    items:
+      $ref: '../../common/RdfValue.yaml'
+    example:
+      - {v: "https://example.com/person/alice", e: true}
+      - {v: "https://example.com/person/bob", e: true}
+      - {v: "https://example.com/concept/quantum", e: true}